Someone started a thread here about "favorite tool" and my response was my Starrett combination square. A combination square is a highly versatile, essential tool for a woodworker. My first combination square was very very cheap, so I should not have been so shocked when I tested it and found that it was badly out of square.
At that moment, then and there, I decided to buy the best combination square I could find. I bought a Starrett, although other less expensive brands may be equally good for less money. Just verify that it is well made and absolutely accurate.
Nobody seems to have mentioned the easy "flip test" for combination squares:
* Find a table, desk, or counter with a perfectly straight edge.
* Check the edge to make sure it is perfectly straight. Use a metal ruler, for example. Verify that your straightedge is straight, too. Flip it around and check again, for example. ;-)
* Lay a sheet of paper down on the table surface next to the edge.
* Place the combination square against the edge with the blade (ruler) perpendicular to it.
* Use the combination square to draw a long fine line on the paper.
* Flip the combination square over and draw a second line very close to the first line.
-> The two lines should be perfectly parallel. Perfect. Really perfect. Not close; perfect.
@Show Soogie Bam
did not say how he was testing his table saw alignment work.
Nobody seems to have mentioned the five cut test to check a table saw for square. There are many different sources and examples on the web. Some seem specific to crosscut sleds, but the general principle is the same. Do a web search for "Five Cut Table Saw Test" to get you started.