You look for a seat that is at minimum the width of the outer sit bone measurement.

That at least lets you know your bones will fit on the seat.

I know I like about 10mm beyond each bone for comfort, but I have very wide sit bones.

You can also measure the length of a seat you like the length of.

Then you have a width and length, and can compare potential seats to those measurements.

If you know whether you like pear-shaped seats or "T" shaped seats, well, that gives you another parameter.

Soft or firm? Sprung or unsprung? Leather or plastic? Cut-out or solid?

Use all that to narrow down the seats you are considering, then start trying seats.

I like 180-200mm wide, 245-260mm long, "T" shaped, firm saddles with no cut-out. Sprung is ok, but not crucial. I like leather cuz i sweat a lot.