It's easier and more accurate to do it after disassembly, otherwise it's much down to your skills in eyeballing.
I'd start with flipping the bike upside down, standing behind it and looking closely at how the wheels align.
If you have a patient friend, ask him/her to help you hold a piece of string tight along the wheels, checking that both front and rear sections of both rims are evenly distanced from the string.

Next thing is to pop the front wheel out, and run a string in a triangle through the dropouts and across the top of the steerer tube. Then measure the distance between the string and the crown of the fork on both sides. That too should be equal.

One tricky thing is if you've hit something squarely head on and managed to bend the fork backwards evenly.
That can change the handling too, but won't come up with these string tests.
Best easy option is to look at the bike from the side, and try to create a reference line aligned with the steerer tube.
Thing is, fork legs usually either sit at an angle from the crown, or have funny curves to them. So unless you have a firm idea of how it looked before, determining the amount of change - if any - can be tricky.