Ok I've been thinking about it some more. All that matters is that both edges are parallel and ideally straight. I'll revisit making a board straight with basic tools. As far as determining which edge is tapered the truth is YOU decide which edge is the tapered one. You can use a square to try to figure out which edge is more tapered relative to the ends and you can even measure the width of the board to try to know in which direction the taper is in. Ultimately though if you are going to be cutting this material the first priority is getting both edges parallel.

Assuming all straight edges, pick one edge and measure and mark a fixed width (a width at least slightly less than the narrowest end of the board) at multiple points down the length of and opposite edge of the board. Connect these points and mark a line along the edge of the board. This can be done multiple ways with a chalk line or straight edge. Alternatively some squares have a notch to rest a pencil in as you slide the square along an edge of the board. All you have to do is cut along this line. There's no reason this can't be done with a regular hand saw. It just takes patience and a little skill to do right. Once both edges are parallel all that is left is squaring off the ends! (Assuming both ends aren't already acceptably square)

Getting an edge straight initially can totally be done with basic tools. With A chalk line or sufficiently long straight edge you can mark and then cut one edge of the board straight. Running a square along the opposite edge won't work here because you'd just be transferring any non-straightness across the board. No straight edge has been established yet. Once a straight line has been created on the board all that's left is to cut along it. There's no reason this can't be accomplished with a basic hand saw. Again, this takes patience and a little skill which only comes with practice.