I have a 97 ford F250 this is the third time I have trouble with oil cooler. But I cannot tell if unit is bad or just rings or seals. How can I check for place of oil leaK ?

Clean area with brake cleaner and observe source of leak.

If you have pulled the oil cooler from off the motor you can do a simple pressure test using a bicycle pump and some water. You will have to seal off three of the four ports and pump pressure into the open one. This should tell if anything is wrong with your unit. If you can't see then try leaving one side empty and look for air bubbles coming from the other side.