I Have A 93 Pathfinder In Which Was In Water Over The Floor Board Thus Submerging The Engine Control Module For A Time. The Engine Will Start, But Runs Terribly. Is There Any Simple Way A Home Mechanic Can Test The Ecm?

Not reliably, no. If it was submerged and had battery power still supplied to it when it got wet, it's most likely a goner. Those things fry themselves when they get wet, usually.