What's to service? undo the velcro, blow 'em up by mouth and leave overnight to see if they leak. Take the gas cylinder out and check for rust, reinsert and make sure it's tight, check all the green plastic bits are in place, repack. Check all the straps are sound and not frayed or twisted.
If really brave (and they're not Hammar (hydrostatic) ones), remove the trigger (the plastic cylinder with the green plug in the end), inflate by mouth and jump overboard to make sure you don't go through the bottom of the harness.
And don't put them through a security airport x-ray, as they contain explosive! I was on a flying trip in the US but brought my own jackets as I was flying over sea, soon to be surrounded by armed police thinkin I had a bomb in my bag!! /forums/images/graemlins/shocked.gif
I recommend the following method for checking automatic lifejacket operation. Find a good anchorage and take crew ashore for lunch. On reaching beach, pull dinghy just a few feet out of the water and instruct crew to hide lifejackets in bottom of dinghy. Go for lunch. On your return, dinghy will have filled with water and, hey presto, any operable lifejackets will have automatically inflated and be floating in bottom of dinghy. Job done