suspension forks
 

how do u know if the suspension forks u buy will fit your bike,do u have to measure the stem

You really need to know several things.
1. What size diameter is your steer tube (probably 1 or 1 1/8")
2. How much steerer tube you want/need (especially if buying used)
3. How much travel you want and your frame is designed for (Changing the distance b/t the wheel and bottom of steerer tube changes frame geometry)