Not sure it you are trying to check for bent handlebars or for bent forks. So I gotta ask.....Did the front tire or forks hit something? Or did the handlebars hit something? ....or both? Inspect the forks for evidence of hitting something if you are unsure if the forks hit something. Forks can get bumped and move the triple tree clamps and make everything appear out of whack and yet the forks are still straight....just need to straighten/align the triple tree clamps.
If you are just checking the handlebars, put the NT on centerstand, straighten the front wheel, loosen the four bolts on the handlebar clamp, position the bar evenly in the clamp left to right. Snug the handlebar clamp bolts. Now look at the bars from a little distance from behind the NT, squat down and look at the handlebar tips. Do they look equal distance to the ground? Measure it if not sure. Rotate the bars full right and full left and check measure the distance from grip ends to the tank at their closest point. If it is more than a 1/4" roughly, the bars may be bent. If there is big road rash mark on the bar end weight, the bars may be bent. It is not easy to know for sure unless you take the bars off and lay them on a flat surface and see the tweeked bars.
If you think the forks are bent, try doing some of that chassis alignment...it should become very obvious at some point.
If you think the forks and the bars may be bent, check the chassis first. If you are able to prove the forks are true & the chassis is straight, then check the bars.
Hopefully someone with serious frame experience will chime in and tell me I'm all messed up and that there is a better way.
Hope this helps. Good luck.