a2psyklnut

What year and model is the bike?

To help determine what you've got take a look at the stem (the thingy that holds the handlebars) and the headset (the bearing assembly at the fork/frame junction. Trying to explain in as simple terms as poss. if too simple please forgive me!

Is there a large nut at the top of the headset just underneath the stem? If so it's a threaded headset. If there are only spacers between the top bearing cap and the stem then it's most likely "threadless"

Another way to tell:
Take a look at the stem. Is/Are there some bolts on the side of the stem? Sometimes there is just one, some two. If you loosen these will the stem move sideways? If yes, then "threadless".

If these are not there and there is ONLY a bolt on the top of the stem that has a long bolt that extends into the steerer tube, then it's "threaded".

Either way, loosen everything up and expose the steerer tube. (i.e. remove the stem/handlebar from the bike.) Measure the diameter of the steerer. A venier caliper is best, but you can use a tape to measure the diameter. This should either be 1", 1-1/8" or if an older Klein or Gary Fisher it could be 1-1/4".

If it's a threadless headset and the steerer tube is 1-1/8" almost all new forks will fit. If not threadless, you will need to buy a new headset and a new stem. I suggest taking it to your LBS if you need to go this route as headset installs require special tools to be done right!

If its a threadless headset and only 1", then your limited in choices of new forks. Marzocchi make steerers in 1", and I believe they're it, unless you find a fork that's on closeout and a couple of years old!

If 1-1/4" then you're screwed!!! You won't find a fork anymore in 1-1/4", but you can get a headset reducer to bring it down to 1-1/8", but then you've got to get the headset and a new stem and the headsets w/reducer capabilities are pricey!

Hope this helps!
L8R