I audition for all kinds of low-budget commercials. The first sign is usually that they're not going through one of the big three casting agencies. But even they occasionally cast some pretty low-rent stuff.
The next sign is the script. Sometimes the scripts are eye-rollingly bad. The casting people will even kinda/sorta acknowledge that you're not exactly doing Aaron Sorkin material and will tell you to just do your best with it.
I had one of these the other day and I realized that you can make just about any cheesy script a lot less cheesy. Or not even cheesy at all. It's kind of a fun challenge. When you see the lines on paper and they're downright wince-worthy, but then you get in there and just give them a totally counterintuitive read, underplaying it or playing it a little opposite or giving the character a bit of winking self-knowledge.
That, of course, may be totally contrary to what the client or the director wants, of course. And that's the third sign of a low-budget production -- a director complicit in producing crapola.
Anyway, I consider it a win-win. If they like my read and hire me then it seems the end-product won't be as bad as the script implies. And if they don't, it means I'm spared appearing on TV in a really embarrassing commercial.