Checking a straight edge for "straight" ....
You can esily verify a straight edge by doing the following. Take your steel, aluminum, plastic whatever you choose to use and lay it on some clean white paper. Make as fine a mark as possible along the entire edge using a knife edge or hard pencil or ball point keeping the same position as you move along. Then flip it end for end keeping the same edge down, and line it up against the line your just made and see if they are parallel. Make another line and see if the two lines are parallel. If not, there's a very good chance it's got a curve to it.
Butting two straight edges together at the store will reveal if either one has a curve, but you won't know which until you try several where there is no gap or light visible between the two them. Flipping one end for end is also required in this process.
Yes, I said plastic because some very fine drafting T squares have an acrylic edge. All drafting triangles are made from some type of clear plastic, so I think their edges should be as good as we woodworkers would require.... you think?
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
Last edited by woodnthings; 11-17-2019 at 12:02 PM.