When I interview, I mention that I have to give my present employer 2 weeks notice, whether it's true or not. Then I carefully watch their reaction. Anything other than supportive is a deal-breaker. Either they have integrity or they don't.
Could you clarify? I'm confused by what difference does it make what they say.
"Of course" - no red flags.
"WHAT?! We expected you to start tomorrow! Can't you just quit? Don't you want to work here?!" - Aaaaand, we're done here.
I have interviewed a lot, and I have literally never experienced anything other than some variation of "of course".
The ones that aren't supportive usually aren't supportive of you quitting without 2 weeks either.
On the flip side, when I am interviewing a candidate I always ask when they would be able to start if we were to make them an offer. A currently employed candidate who says "tomorrow" raises huge red flags.
Tell them 3 weeks. Get escorted politely off former employer when I give notice (its just policy, good luck). Vacation!