Two years ago a friend turned me on to the LightKeeper Pro - an electronic repair tool for mini-light sets. Battery operated it checks for power along the string (blown fuse/bad bulb), has a single bulb & fuse tester, and best of all makes "repairs" to the shunt resistors in the base of the lights to light the string and show you the bad bulbs. Easy to use, good instructions. found a new "Where to Buy" page from their main site. I got mine at Lowe's for less than $20 - one of the best buys I ever made. I generally buy the strings I need plus one, and use that "extra" as a source of replacement bulbs so the sockets match, etc. Here's the link:
http://lightkeeperpro.com/wheretobuy.asp