How do you know what a care package contains in Modern Warfare 2?

According to the Call of Duty wiki:

If a UAV or Counter-UAV is dropped, leave it for a teammate as this will award the "Share Package" XP bonus while having the exact same effect, as UAVs and Counter-UAVs apply to the whole team.

How do you know what the air-drop contains without taking it?

To expand on what Arda Xi said once the package lands it should show an arrow with an image inside. Here are most of the images that could show up in a care package:

It isn't included in the list below but another possible drop is an ammo resupply crate.

1. when this shows up the care package contains has a UAV

2. when this one shows up it is a counter UAV

3. this means the package contains a sentry gun

4. this is a predator missile in a box

5. this is a precision air strike

6. harrier strike anyone? (I'm not sure this is the right picture...)
7. attack helicopter is contained within
8. a pavelow
9. stealth bomber...
10. Chopper gunner (Better pick this one up)
11. Ac-130 controls in this box
12. EMP strike waiting in the box

Those are all the possibilities for a care package. All of this information and all of these pictures were gotten from the Call of Duty Wikia