How do I know the air-speed of my kart?

The chart JohnoBoy posted is close, but uses the wrong units. Super Mario Wiki's page on MK7 has all the values. Here's the short version for airspeed only:

1. All characters have a base airspeed of 4 (out of 6).
2. If you're using one of these karts, you get a -0.25 penalty:
   • Barrel Train
   • Blue Seven
   • Bolt Buggy
   • Bruiser/Growlster
   • Cact-X
   • Gold Body
   • Tiny Tug
   • Zucchini/Gherkin
3. If you're using one of these karts, you get a +0.5 bonus:
   • Birthday Girl/Royal Ribbon
   • Bumble V
   • Cloud 9
   • Egg 1
   • Pipe Frame
   • Soda Jet
4. If you're using one of these wheels, you get a +1 bonus:
   • Roller
   • Sponge
   • Wood
5. If you're using one of these gliders, you get a -1.5 penalty:
   • Paraglider/Parafoil
   • Swooper/Swoop
6. If you're using one of these gliders, you get a -3 penalty:
   • Flower Glider
   • Peach Parasol

Note that gliders have the strongest influence on airspeed - also note that the gliders with the biggest airspeed penalty have the biggest air handling bonus, so they're not total step-downs.