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Very often a part has two or more surfaces that are meant to be at right angles to each other.

The solution to this is to mount the part on one of the surfaces touching a surface plate. The other surface will now be at right angles to the first. The simplest way of testing this is to place a square on the surface plate and push it against the vertical surface on the workpiece.

If a light is shone at the point where the square touches then it is easy to see even the smallest gap. This would denote that the surface of the work piece is not straight or is not at aright angle to its base.

It is possible to measure the gap using a feeler gauge. But this is only going to work for larger gaps.

If a more sensitive method is required then there are devices such as the Indi-Square and SquareMaster. These appear to be very solid triangular castings. The bottom is flat and on the vertical side they contain a a sliding mechanism that holds a sensitive DTI which can easily be slid up and down against the surface being tested. This will give an actual measurement of the differences as it goes up and down.

Mitutoyo also make a similar thing they call Square Master. This can be bought on Amazon for about $6000.