How do I find out the speed of an Ethernet cable?

If the cable has 4 twisted pairs (Orange, Green, Blue, Brown) then it's capable of 1gbps link speeds, provided there's a gigabit adapter/switch at both ends. Cable 3 is a Cat5E, which is definitely capable of gigabit speeds. The other cables are, most likely, also Cat5 or Cat5E.

Whether you get 100 mbit or 1000 mbit speeds is really dependent on the Ethernet adapter at each end. If you have a 100 mbps switch then you'll only get 100 mbps speeds even if the cable is Cat6. As long as you have 4 twisted pairs you should be able to get gigabit speeds from gigabit adapters unless there's a physical problem with the cable.

Most of the markings on ethernet cables refer to safety certifications, standards bodies ratings, and similar things that won't much help much. The few things you might want to look for include:

Useful Markings

Category: CAT.5, CAT.5e, CAT.6 etc

As others have mentioned, the category listing is going to give you the most information. For modern data networking, you want at least CAT5e.

Be highly suspicious if there's no category listing.

More on categories below.

Category class: ENHANCED

This is sometimes used to indicate CAT.5e.

Pairs: 4PR, 4PAIR, 2PR

Indicates the number of pairs of wires. Modern ethernet cables are 4 pair (8 wires) but I have come across a CAT5e 2PR that was slowing down a network.

Shielding: UTP, STP, FTP, S/UTP, F/UTP, S/UTP

Indicates the type of shielding, which can protect from interference. Unshielded (UTP) is fine unless the cable will be run through a particularly high interference environment like through the walls.

More on shielding below.

Wire Gauge: 24AWG, 26AWG, 28AWG, 30AWG, etc

Indicates the thickness of the individual wires. (Thicker wire = Smaller number.) 24AWG and 26AWG seem to be standard. A thicker wire might indicate higher bandwidth / speed capabilities as well as greater durability. I'd look somewhat askance at 28AWG/30AWG.

Speed: 350MHz, 500MHz, etc

Indicates manufacturer claims as to bandwidth performance. Should be taken with a grain of salt, as they're not always tested.

Cable Categories

Below is a summary of the ethernet cable categories and the preformance standards and bodies that apply to each.

Note, the bandwidth ratings per EIA/TIA are specified as up to the number listed. (As a range from 1 to x MHz.) However, they are in fact the minimum requirement for each class. I find this confusing, but there you go.

category    bandwidth   shielding      speed         EIA/TIA     IEEE        ISO/IEC 11801  use/notes
            (MHz)                      (Mbps)                    (BASE-T)                               
                                                                                                         
  CAT3      16          UTP            10            568B        10          Class C        voice only
  CAT4      20          UTP            16                        10/100                     obsolete
  CAT5      100         UTP            10/100        568A        10/100                     obsolete (-> 5e)
  CAT5e     100         UTP            10/100/1000   568B2       1000        Class D        networking minimum
  CAT6      250         UTP/STP        10/100/1000   568B2-1     10GB        Class E
  CAT6a     500         STP            10,000        568C.1      10GB        Class E-A
  CAT7      600         S/FTP          10,000                    10G         Class F
  CAT7a     1,000       S/FTP          10,000                    10G         Class F-A
  CAT8.1    2,000       U/FTP, F/UTP   40,000        568-C.2-1   25G/40G     Class I        data centers    
  CAT8.2    2,000       F/FTP, S/FTP   40,000        568-C.2-1   25G/40G     Class II       data centers

CAT8.1 and CAT8.2 standards are under development.

Shielding Codes

These codes indicate shielding material to reduce electromagnetic interference. If present a screen is wrapped around each individual wire and/or the whole bundle of wires.

 code    meaning                            wrapping       wrapping        
                                            (all wires)    (each wire)     
                                                         
 UTP     Unshielded Twisted Pairs           none           none            
 FTP     Foil Twisted Pairs                 none           foil
 STP     Shielded Twisted Pairs             none           braid
 S/UTP   Screened Unshielded Twisted Pair   braid          none            
 F/UTP   Foiled Unshielded Twisted Pair     foil           foil
 S/FTP   Screened Foiled Twisted Pair       braid          foil

Markings, Decoded

Here are some of the markings on cables I had laying around and their meanings.

E21220 Я⅃ AWM 2835 24AWG 60°C 30V TIA/EIA 268B.2 UTP CAT.6 RAPID CONN

• E21220: Cable Extrusion Certification from UL (Underwriters Laborites)
• ЯU: UL (Underwriters Laborites) Recognized Component Mark
• AWM 2835: UL AWM Style "Multi-conductor, thermoplastic insulation and jacket"
• 24AWG: 24 gauge wire
• 60°C: Temperature rating
• 30V: Voltage rating
• TIA/EIA 568B.2: ANSI/TIA/EIA Telecommunications Standard
• UTP: Unshielded Twisted Pairs
• CAT.6: Cable category 6
• RAPID CONN: Manufacturer

Я⅃ AWM 2835 28 AWG/2PR 60°C 30V VW-1 ETL VERIFIED TIA/EIA-568B.2 CAT5.E UTP

• ЯU: UL (Underwriters Laborites) Recognized Component Mark
• AWM 2835: UL AWM Style "Multi-conductor, thermoplastic insulation and jacket"
• 28 AWG/2PR 28 gauge wire / 2 pairs of wires (4 wires total)
• 60°C: temperature rating
• 30V: voltage rating
• VW-1: UL Vertical Wire Flame Test (UL 1581)
• ETL VERIFIED: Preformance Certification from Intertek
• TIA/EIA 568B.2: ANSI/TIA/EIA Telecommunications Standard
• CAT.5E: Cable category 5E
• UTP: Unshielded Twisted Pairs

ENHANCED CAT.5 UTP 350MHZ CM 75°C 4PR 24AWG VERIFIED (UL) E201403 CSALL109448 ETL VERIFIED TO TIA/EIA 568-A PATCH CABLE

• ENHANCED CAT.5: Cable category CAT5e
• UTP: Unshielded Twisted Pairs
• 350MHZ: Bandwidth
• CM: [Cable meeting UL 1581, Sec. 1160 (Vertical-Tray)][CM]
• 75°C: temperature rating
• 4PR: 4 pairs of wires (8 total)
• 24AWG: 24 gauge wire
• VERIFIED (UL) E201403 CSALL109448: not sure, probably a UL standard
• ETL VERIFIED: Preformance Certification from Intertek
• TO TIA/EIA 568-A: ANSI/TIA/EIA Telecommunications Standard
• PATCH CABLE: ethernet cables are a subcategory of patch cables

I'm not an expert in this field, I just spent a bunch of time googling. Errors are entirely possible.