Alf: If you ain't happy with the '2 thou tolerance' type stuff, which I agree is a little intimidating, try going back to the 'popular woodworking' style approach of using the tool itself to assess its accuracy (as in how to check a straightedge):
It's all about right angled 'shoulders'... Cut one with any method you're happy with (bandsaw, TS, handsaw...), on a 'reasonable furniture length' piece of stock (let's say 2 feet long, for a medium size table apron) that is at least 6 inches wide. Then true it with the plane under test. Now run the thing lengthwise through the TS or bandsaw to give 2 identical pieces (the stock, not the plane

)
Flip one of the 2 pieces upside down and mate them. (ooh - fun!) If there is obvious 'non-parallelism' along the length of your 'shoulder scarfed' 4 foot piece, the plane is unacceptably out of square. If it is 'eyeball acceptable', then the plane is useable... Ah, the joy of basic physics and euclidian geometry...