You can go to an airsoft field and use a cronograph, a device that measures the FPS of the bb's. In a pinch you can also shoot a soda can at 5 feet, it should pierce the front side but not the back in order to be safe and fun.
See the related links for cheap airsoft guns.
the store
just ask your local gun shop or airsoft dealer
http://www.airsocom.com/Airsoft/Replica_Models/Airsoft_M14/ right side
online. easy.
An Airsoft gun
look up an m4 airsoft gun clip on airsplat for around 10 bucks
A pump airsoft gun is a gun that has to be pumped every time before shooting. Ex. Airsoft shotgun
Go to the Google website, and from the upper toolbar select IMAGES. On the search page, enter "airsoft" or "BB gun", and you should find hundreds of pictures.
No
only one way to find out
There are plenty of p90 airsoft guns out there. I recommend searching "airsoft" in google maps. Once you find a store, check their website for the gun of your choice. Hope I helped.