Old school mechanics kept a piece of granite, marble, or a glass pane to check forks for squareness.

Lay the fork on a flat surface with the crown, or blades just below the crown against the corner, and the tips on the surface. If all is jake you'd have 4 point contact, like an OK table. OTOH if the blades aren't squared up, it'll rock like $5 card table. Note the bold If, this isn't a bullet proof test because there are other variables, like front edges on dropouts that aren't perfect. But it's a reasonable place to start.

Next you need to check for side to side deflections, and/or whether one dropout is higher than the other. Here's a way to start. Mount a QR axle into a 2x4 so you can clamp it above a bench vice, or use one of these. Put the fork on making sure to seat fully to the top of the dropouts, and with the steerer leaning on the bench. Mark the position, repeat with the fork flipped. A good fork will end up in the same place.

Unfortunately if the fork fails, it might be a high dropout, or a deflection of both blades to one side. It's very hard to separate these because they cause similar effects, but sometimes a good eye can tell you. Yo help your eye, put a straight from dropout, past the crown and out past the end of the steerer. Measure the horizontal distance at the end of the steerer. Compare to the same test on the other blade.

When finished cross check on that sheet of granite.If you don't have any flat glass, you can do this in a cemetery, the same way I had to in Vermont after a crash 40 odd years ago.