I'm making a lemon bars/squares recipe that I've made a few times before, and I keep having trouble deciding when to take it out of the oven. I don't want to overbake it, but I don't want to underbake it either as I won't be able to cut it properly into squares. The recipe says "until set, about 25-30 minutes". My lemon bars have been in the oven over half an hour; the top has gone light brown, but it still jiggles around like liquid when I move it. How do I know when it's "set"?
Custard is always sort of jiggly, even when it's "set"; furthermore, it's one of those dishes that sets properly only after you remove it from the heat of the oven. The link you gave in the comments mentions a thumbprint test for doneness:
Next time try that, and see if it sets to the desired consistency when cooling. Browning is not usually desired for this type of dish, so I'd say 45mins is too long :)
Note: The part that still baffles me is that you mentioned it still being liquid when cut into. That's definitely not supposed to happen, and I'm not sure what went wrong there, so perhaps try a different recipe next time?
Custard without any flour or starch should be baked to about 83 Celsius internal temperature. Below 80 is underbaked, above 90 it curdles, so you have a very small window of good texture. It may be even narrower, it depends a bit on the other stuff you have in there, sugar gives you wiggle room but acids (including lemon juice!) will make it harder to get right.
If you have starch or flour, you need to reach almost boiling temperatures. 95 Celsius should be OK.
If you want high confidence, you really need to measure the internal temperature. External signs are not reliable, and time even less so.