you can use nitrogen

You can pressure test with shop air (needs to be vacuumed longer and best if the air is VERY DRY). I made up a set of fittings to connect to one hose on the manifold set with a ball valve to shut it off. Vacuum will not necessarily show all leaks as stated above.

As others have posted above,
Dry Nitrogen is the conventional Way to pressure test, and the EPA states in writing that “small amounts” of Freon can be added to the nitrogen gas for leak testing.

If it is an old R-12 system, some pressure test with
Difluoroethane, the relatively cheap gas in some “Dust-Off” or “Canned Air” spray cans.

In the AC world Difluoroethane is known also as HFC-152A
It is flamable.

You need to buy a “Can Tap” to transfer the gas from the Dust-Off cans through a hose to AC systems.

Some R-12 system conversion details using HFC-152A are here:

https://www.s10forum.com/threads/how-to-r152a-conversion.493953/

Feets, how long since the last charge? If it still has pressure after 10 years you should be fine.

I've also used shop air.

Well, since this topic was started in 2012, lots of things may have changed!

Best way to test is to pressure test with a nitrogen/hydrogen mix. They will charge the system at about 200 psi and then use a sniffer which will detect the hydrogen leaking from any seal or fitting. Sniffer will indicate the level of leak in a gram or ounce per year scale. Vacuum alone will not be accurate, shop air is just adding moisture and there is no way to sniff for leaks. If you want to keep R12 or not waste R134 which is getting more expensive, find a shop that has this setup. Save yourself future depression and find and fix the leaks now.