Need help understanding how to diagnose F/P

Need help understanding how to diagnose F/Pump. '91 Aerostar. I have had the fuel pump replaced twice in the last year (used pumps) and it seems I need another. I am unwilling to have anyone else do it for me again! But 1st I must make sure it is the pump. It is not out of gas. The roll over switch hasn't been triggered. The relay clicks when the ignition is switched on. The pump doesn't prime up but never did when it was replaced 4 months ago either and it still worked fine. I will replace with a new pump this time if required...:banghead:

Could someone give me some pointers, please?