It's hard. Because everyone claims to be doing TDD, just asking in the interview "do you do TDD" is pointless.

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.