When's that "sound" an bad tube?

I'm real new to the tube game, but I've now got a tube pre-amp and a tubed source, so I guess it is about time I asked the inevitable question -- how do you know a bad tube when you've got one? I've been through a whole array of uninvited sounds from the preamp (a VTL 2.5) but today I got a new one. Sounds like static or crackeling, more than anything. Intermittant, irregular, varrying volumes, often silent, and only from the right speaker. It's not source dependant, so we can rule that out, and the amp is quiet as a churchmouse--so all fingers point to the preamp (which is a noisy little beast, it hisses, sometimes even hums, and now it appears to have caught a case of sporadic crackeling--what next?). So, as I haven't owned tubes long enough to have had one go on me yet (unless this is it) the tubes are immediately my first suspect. (And besides, this sounds kinda like what I would imagine a failing tube to sound like). My question? How do I (short of going out and getting a new set of tubes, switching them out, turning it on, and seeing if it still crackles at me) know whether I've got an ailing tube? Oh, if it helps, the tubes have no more than 4-500 hours of use on them (I think life expectancy is somewhere around 3,000?). Also, if anyone's got a good URL or reference on the care and feeding of tubes, it would be a great help. Many thanks.