This may sound stupid but I just bought a 1990 toyota pickup and believe it to be lifted it has a set of 31x10.50R15 on it on 15x10 rims. Is there any way to tell if it has been lifted? And if so how much?

It should be a leaf spring rear. Check the spring hangers (shackles) and see if they are a different size than one you KNOW to be stock. Also look for a lift block between the diff and the springs.

1990 is IFS, so there would be a bunch of drop bracketry dropping the lower control arms & diff housing down.

If it has a bodylift, then you'd be looking for spacer blocks on the bodymounts and under the bed.

1990 is IFS, so there would be a bunch of drop bracketry dropping the lower control arms & diff housing down.

If it has a bodylift, then you'd be looking for spacer blocks on the bodymounts and under the bed.
What he said.

And no lift is needed on those models to fit 31's. Are you sure those are 15X10 wheels. A 10" wide tire is awful narrow for a 10" wide wheel.