I'm planning to upgrade the suspension fork on an older mtn bike to something new enough to have disk mounts. I'm trying not to dump too much money into the project, so I will be looking at swap meets for a fork.

Question is, how do I inspect or what do I look for to make sure I am getting a usable fork?

Thom,

Look for the usual stuff.
1)What do you want in a fork- how much travel, how much adjustment (compresion damping, rebound damping adj , preload adj,, travel adj -maybe can have 120 or 100 mm of travel)do you want actual damping, or do you just want a cheapo sprung fork(you don't want just a sprung cheapo wal mart bike type fork, but just covering all the bases).

2) Brand specific-Rock shox, Manitou etc? Name brands are easier to find parts for.

3)Price
4)Weight- all air usually 3.5 or less-metal spring about 4 lbs.
4) Defects-rust, corrosion, leaks, broken knobs adj , scored up fork sliders etc. Decent length steerer. What size 1" or 1 1/8" lots more selection on 1 1/8.
Make sure it has enough steerer length for your frame- VERY IMPORTANT- too long is no problem, too short is death-useless.

It is much easier to really check a fork if it is on a bike.
Too bad you don't live next door.I would sell you a Manitou Black elite120/100 with all sort of adj- 8.5" steerer for $85. Low use, low miles no real trail use.

Just don't be in a hurry to buy. You can bounce a bikeless for up and down to check to see if it still has damping-not as good as riding, but better than nothing.

First and most important is to look for any dings on the swept portion of the legs. Any such thing will eat seals quicker. Then there's the steerer length as mentioned above. And finally if the fork is not on a bike grab the stanchion and leg and try to wiggle to see if the bushings are still good with no free play that would allow the wheel to kick around a little.