There are a couple of ways a ruler can be "off"
1) It's worn down in the center - to check/notice for that -
a) when you start cutting strips - you thought you cut correctly - and when you move the ruler down for the next cut, the fabric isn't lined up with the measuring line on the ruler
This is when I finally figured out my RULER was the problem - it seemed strange to me that cut edge was not straight when I knew I hadn't moved the fabric after cutting it.
b) put the cutting edge against a counter or surface you know is straight - does the whole edge touch the surface?
2) It was printed "wrong" - I think I go by "marjority rule" - sorry, bad pun - and see if it lines up with several other rulers -
Offhand, I can think of at least three that I've had were "off" - either in actual size (a square) or where the lines were printed.