Can you tell from the impeller?

No. High flows have a plate over the impeller or I should say an impeller enclosed, having sides on the fins.

Thanks Everett. Just ordered this pump http://store.summitracing.com/partdetail.asp?part=MIL-16301 with this pulley http://store.summitracing.com/partdetail.asp?part=SUM-G3959 I'm going from a 7" pulley to this 5 5/8" pulley. If this won't do it I don't know what will!