Starting any woodworking project requires good wood.
Knots, twists, and other damage to lumber will mess up your project before you even get started.
Inspect boards looking from one end to the other and note any warping, twisting, or splits.
Pass on those that arent straight.
Then look for knots or other irregularities along the wood itself.
Damage from rough handling can also render lumber unusable.
When inspecting plywood, again check for warping or bowing due to uneven weight distribution.
Bent wood wont saw properly so avoid it.
A good rule of thumb is to not buy the last bit of wood from any pile.
How to Pick Materials for a Workbench| This Old House