How to determine what applications are using what features on Android?
All Android applications have to be granted 'Permissions'. These 'Permissions' include internet access, GPS, etc.

You can see these 'Permissions' by going to the application in the market, hitting 'Menu' and selecting 'Security' (I think).

As for 'activating features', applications cannot activate GPS or WiFi if you have them turned off. Also, GPS, even when activated, is only 'awake' when an application is polling for location, then immediately goes back to sleep.

It's always good to remember to turn off WiFi and other features you don't need when you aren't using them, although Android is very good about only using these features when necessary.