Rail question (square)

Sorry if this has been addressed before. How do I know if my rails are square? I get some odd squeaks every now and again when the heads are printing in certain areas. I have cleaned the rails and lubricated everything with nlgi-1 super lube. Checked belt tension. Covered the top where the cable chain rubs with Teflon tape. Filament feed tubes run straight up out of the hot end so no more rubbing on the sides. After all that the squeaking has been greatly reduced.
Just want to verify my rails are straight and square so I can rule that out. Trying to prevent premature wear.

Next I’m going to replace the drive gears.


The squeeking sound that came from my printer was related to friction between belts and pulleys. It dissapeared after i smudged a teeny bit of grease on the belts. Unfortunately I dont have any good tips on veryfying the straightness of the rails. But I can tell you that I replaced my x-rail yesterday and the old rail or carriage was total garbage. Lots of wiggle…

I’ve loosened the linear rails and readjusted them with some shims I’ve printed bevor according to this post: