well the way to tell if they are really sqaure is by using the inside ruler and making a 3-4-5 triangle, or a right triangle, measure between the 3" mark and the 4" mark on the other side and if it is 5" you are still square, otherwise you are not.
as for getting them back in square... never done that
What Chaos said is probably the best way to check. 3,4,5 or 6,8,10 or 12,16,20 if it's a big framing square, The larger the dimensions the better able you'll be able to see if it's out of true or not. The only adjustment I know of is holding the square like a gun < with the short leg in your hand> either right side up or upside down depending on whether you're Obtuse or Acute < well not you, but the square, I'm always Obtuse>
and whacking it,firmly on the table, gently yet evenly along the entire inside or out side edge, soas to not cause it to buckle in the middle. If two whacks are not enough to get it square, throw it away, or cut it at a 45 on the inside corner and make yourself two new straight edges.
Both incorrect, and do either of you ever attempt such a concept on a framing square - or have either of you ever done this? Ever test your framing square on the jobsite than make it square? That's the test of a skilled traidsmen beyond say owning stairway stops for it. The ability to take a framing square, check it for square, adjust as needed than continue functioning as opposed to tossing it asside and sending a runner to the store for one that will hopefully because it's off the shelf true and square.
No offense hopefully to either, most at best no doubt had your opionion but didn't have the guts to post. Others without an idea didn't post in being silent majority but hoping to learn without any incentive by them to go further in this post or at least say something...
Anyone ever heard of an "end for end" test that both works with a level in seeing if it's the same no matter which side is left or right, and given a pencil for a square, reversing it and drawing a line on plywood from each side?
345 method works for a wall but doesn't allow for tollerances that 3"+4"=5" has to do with a 18"x24" framing square. We are what measuring for accuracy within the first 1/3 of the square and trusting that that's sufficient based upon some measurement that works best over a few feet of measurement verses within a few inches of noting a difference?
Incredibly important to ensure your framing square is really square, that as with your level in a way we had to be yearly tested on in the military for our gunner's levels, and certification that we could ensure they were calabrated correctly.
Anyway for the framing square, one should do more research into what side of the corner of the level to hit as it kind of kinks it into one direction or another in getting it back to square. Got this bronze colored framing square I have had for 20 years now. It's a part of me and square. Would rather correct than throw it out and it's cost into the trash as if a store bought one is as a given also square.
How to correct for and adjust such a level? How to know if it's not quite square... that's a carpenter trick that should be known and easy to test than adjust for.
I've always been told the way ship describes, draw a line with the square, then flip it over and draw another. if the two are one and the same, you're good to go, if not then you've got a problem. In this case, the longer the square, the more accurate you can get it. You of course need to use a sharp pencil for this... And it relies on whatever you are drawing on having a straight edge also.
Calibrating levels - I'm assuming that we are talking about spirit levels and those of medium - good quality. Most of these have little crews that you loosen and you can turn the spirit capsule so that it's level again.
Cheap levels tend to skimp on these and hence probably the best place for one of those that has stopped being level is the nearest refuse receptacle...