Often in bookcrafts boards ~ for covers ~ come in pairs. There is a very quick way to check that the two are identical, and accurate as regards their squareness. This must be done at an early stage. If anything is the slightest bit out of true it will show up ~ usually by spoiling things ~ and wasting a lot of work which can no longer be rectified.
Take the two boards and place them ~ especially if they are one sided ~ side by side and in the finished orientation. This means that if the boards have an outside and an inside then the combined bundle is to show two outsides. Tap them down ~ line them up ~ check that all edges are identically aligned. Preferably not even a fraction of a millimetre of difference anywhare. This checks that all the dimensions are identical on the two boards.
Now take one board and turn it over around the vertical axis. Place them together ~ tap down ~ check alignements. If anything is out of square then an alignments error will show. The amount of the error will be doubled, and so this is a sensitive test.
If rectification is required it must be done now. Using a reliable set square ~ try square ~ T-square ~ check every corner of one board. The error you are looking for will be half that noticed earlier ~ possibly almost too small to notice. If you cannot find it check the other board (and wonder why your very first test showed everything to be OK. Once the faulty corner is found trim it into squareness. It will alter the dimension slightly. The other board will have to be trimmed accordingly.
It is very difficult to trim small amounts away from boards. A bookbinder faced with a problem of unequal boards will make a fresh pair to the exact size. If economically minded ~ and with the space ~ he will mark them as unsquare and put them in a pile ready for re-use on a smaller job.
I cannot say it often enough. If there is a small error ~ larg enough to benoticed by you ~ then it must be rectified or rejected. Do not try to bodge on.