No Square?.... No problem.

This is a bit old school, well ancient really, but handy to know in a fix.
Before set-squares were checked to the nearest nanometre and proper saws had a wooden handle without a plastic square edge, craftsmen knew several ways to make sure their cut, and their work, was at right angles.

The above technique employs a simple compass, which can be made from anything - even a bit of string- and a straight edge to draw a line. When you see it, it's stunningly simple, if not obvious, and can be scaled to any proportions - handy on a building site too.. 
Indeed, most of the greatest historical buildings in the world, from the pyramids to Chartres Cathedral and the like, were set out and built using simple techniques such as this.

So, if you can't find your try-square or your laser turning thingy-ma-bob,  it's fine. 
Try this - you'll be in good company.