How to know when to remove lemon squares from oven?
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.