I'm gonna start pontificating here - since I'm most the way though a pint of Clint's Belgian.
I could go on about diacetyl, VDKs, infections, esters and whatever else, but I don't think that's really it - usually when you can pick those out individually, it is pretty obviously crap already.
After brewing, and drinking several hundred batches, good, shit, and everywhere in between, along with a lot of different commercial beer, you get a sense for it in the patterns of flavour - in that beer you didn't like as much as the other one for mysterious reasons, or in that other beer that, while having an otherwise completely unremarkable recipe and process, is still remembered among your mates as a legendary beer years later.
I don't think my palate is particularly good, I'm pretty lousy at picking out individual flavour notes explicitly, or coming up with various fruits and vegetables to compare hop varieties to.
But I've brewed an infected beer, which was infected with exact same greblie as that one Neanderthal brought to the case swap, for example, so the relatively subtle infection-flavour that beer had, can be easily matched to the more prominently infected beer I badly brewed some years ago. So I guess that same concept carries to all the other bad flavours - having had them before in crap beers, I now have that memory of the flavour which can be compared.
So to summarise - if it reminds me of one of my previous fuckups, it's a bad ferment.
