Try this:
Final speed minus Initial speed (both in m/s) divided by the time between them (in seconds) will give you the acceleration in meters per second squared. Divide this by 9.8 to get the answer in g's.
So if you have 0 to say 30 m/s in 2 seconds, thats (30/2)/9.8 = 1.5 g's
Hope that helps.