Ask questions about their testing approach. Remember, no company is perfect. Often, small startups decide it's in their best interest to rely on manual testing and accept that their products are buggy. This can be an opportunity if you're able to clean things up, though.

If a company is honest, and takes the time to answer your questions, then it's probably a good sign. If the company thinks you should be grateful for a job and grovel for whatever crumbs they drop, RUN!

Remember, though, no company is perfect. It's better to work for a company that has an imperfect TDD then one that treats you like dirt.

Example. Last year I consulted at an IC company, building a huge "moon-shot" chip. You might expect TDD to be an absolute certainty in such an operation. Not so, individuals had all sorts of justifications for not doing TDD.

Here is one idea. In the technical interviews, when the inevitable "and do you have any questions for me?" moment comes up, ask them to talk about their best test.

If they have a good answer, it's likely you've found somewhere that genuinely values TDD.