Blow them up and if they go down within 24 hours discard. Otherwise refold and make sure the gas cylinder is screwed in properly and not covered in rust.
How's that for value, speed, and efficiency? /forums/images/graemlins/smile.gif
Having visited a liferaft, and lifejacket servicing establishment. It is definitely a DIY job, unless there is some reason why you need them serviced 'professionally'
All the servicing places do is. Visually check for wear on harnesses, straps and the lifejacket itself. Remove gas cartridge, visually inspect and weigh it, replace if it is corroded in any way or it's weight is incorrect. Test that the manual lever works smoothly, without the gas catridge in. Inflate the lifejacket, preferably not with the mouth, and leave inflated for 24 hours. Deflate, and repack, and mark on the tag inside the date that you serviced it.
In addition to what malcb has said change the "bobbins" that activate the firing mechanism - they tend to absorb moisture & weaken. If you cant find them down your way try www.suffolk-sailing.co.uk/main.htm - (no connection other than satisfied customer etc...)
Talking to lifeboat crew some time ago, they suggested putting a condom around the cylinder, because one of the most common failures is the canister chaffing a hole in the jacket!!