Use a high powered flashlight or even an auto garage-type, corded trouble light, and just lay it on the floor and move it around to scan for scratches. You don't need to get your head down low. It helps to do this when it's dark out as you'll have good contrast then.

You could use some solvent/thinner for the stain to wipe on a thin coat. That usually reveals scratches that can be sanded out prior to applying the stain. After you get the surface prepared the way you want it, the stain should yield a consistent color.

A fluorescent trouble light (single straight tube) can be placed on the surface and then tilted to provide almost a slit with the light washing across the surface.

Defects stand out very well.