What you really need is one square that you use as a master square to check your other squares. Keep your standard in a separate box or protected place. Take it with you when you go to purchase any new square. You can find lots of measuring tools that are surprisingly accurate if you match them against your known accurate master square.
So, what to use as your master square? You can spend a $100 or more for a Starett or Brown & Sharp machinist square which goes out of square the first time you drop it or you can go to an art or drafting supply store and get a $10-12 plastic 30-60-90 drafting square. To prove it's exactly 90°, take two to a glass counter, put the shorter legs on the counter and face the longer legs at each other and butt them together (like a teepee). If the legs exactly butt, you have two perfect 90° angles. Buy one of the triangles and that becomes your standard or master. Now you can check your other working squares--or any you want to purchase--and determine if they are accurate. Keep your plastic master square some place where it doesn't get banged up and you will always have a perfect reference.
Use inexpensive squares as your working squares. Check them from time to time. It they get banged up and out of tolerance, cheaper to replace the inexpensive ones than the high priced machinest square.