How to square off off 90 degree cuts in 2x4?

Watch a youtube vid, like the ones made by Shawn Graham (wortheffort woodworking) he shows how to make a workbench without a bench and no fancy tools.

Do you have electricaI power?  Get electrical power and water and get it into a building/shop, before you build a work bench.  Invest in some tools.  You don't need a work bench unless you have some tools to work with.  Buy the tools first and work outside or on the floor.  Work yourself up from there.  Look at work benches at Home Depot, going the route you are going.  They don't cost anymore than what you will build, if you have to buy the material, etc.  To answer the question.  If you have no tools, such as a square, fine something that is square, like a brick or a board that has a factory cut on one end.  If the board is not too warped, it will be square enough. Finished wood furniture has excellent straight lines and right corners.  And of course, there is your eyeball. 

surform rasp works for small amounts.  however, it is usually better to make careful cuts in the beginning or change your design or method.  Work bench?  Why would this need to be perfectly square?  Suppose you built the top with all the 2x4 a couple of inches longer than design and then use circular saw to cut all of the ends off at same time?  [I have an adjustable saw guide that'll clamp onto the top to be cut [or any surface up to 48 inches wide] -- carpenter's square to make sure i get it on square before i use my saw.]  What tools do your neighbors have that might help with this?  -- grampa