a calipers ?
Squaring all side of a piece of wood, each side square to it's immediate side, and sides opposites being parallel requires a jointer, a planer, and a table saw, at least that's the way I do it and I know of no other way.... just sayin'
You have to start with a straight, flat surface or side, not an edge. Once that's done, you joint the edges on either side using the previously created flat surface against the fence. Then it's on to the planer to make it a uniform thickness. Then you can rip it on the tablesaw, just remember which edge you registered against the fence.
Tolerances...? as close as you can possibly get to zero... 1/64th, 1/100 of an inch, just do your best. Rough carpentry tolerances are not close enough for woodworking. We strive for "no gaps" and sometimes we are good enough/lucky enough to get there. A calipers will only measure thickness, not all that useful for woodworking, but you will need them on occasion. JMO.
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)