if it's over the internet there's absolutely no way to determine if it's a lemon or not. Brand new instruments from word famous brands (yes, Fender, Ricks, Warwicks) can be bona fide lemons from the start on, it's inherent in their designs. Say SOME Gibson EB basses or other old "nice try" experiments. They're too thin and slim and sleek designed. Also, what's a lemon for you ain't a lemon for me. I check for BALANCE, that it doesn't neck dive at all when sitting or standing, so most new basses regardless of brand have great deal of "lemon" in them if they neck dive. And if I am going to be real picky about it, most Fender J and P basses between 1965 - 1985 I consider lemons. Now, however, to most people this is a non-issue, and a too prickly stance to have.
If it's an second hand ad, and I am able to travel to try it out before buy, I can feel it within 30 seconds of playing it. Unamplified. Like most people here already have posted about, dead spots I know where to hear for, neck relief, and if the truss rod's stuck or not. Actually, "playability" is just 50 percent. The electronics, pickups and so on can always be changed out, but then I'll spend more. I don't want to end up having poor pickups laying around, that no one will ever buy. By the way, my interpretation of lemon is that a bass was sore from the start on, that not anything can be done to it afterwards. You can always do a re-paint job, change pickups, de-fret and re-fret, adjust truss rod, solder pots inside, screw output jack back, and so on, screw new strapholders in, etc etc. Lemon, to me, is when it's beyond remedy from the start on. Like if it has been stored in a damp environment and the neck will never come back whatever you do. If I detect that the seller doesn't know what I am talking about, and not even be able to detect these things, I am bargaining down that much, so I can only use it as a spare part thing, i e the hardware. But I bet you, one hundred percent, the seller just wants to pass it on to the next potential buyer that doesn't have THAT much nitpicking and hairsplitting opinions, and can't find that many faults with it.