That is really a huge question since there is such a long history for the P38
but I will try to help. First, there is almost always some numbers representing the date on the slide. War timers have a 2 or 3 letter code followed by the date. Post War P1's have a date and usually a date stamped on the frame, go by the date on the slide. Commercial p38's I am not as sure on, but they should have a proof date just before or after what looks like an antler symbol. If it is a WWII gun (or any other for that fact) make sure the serial numbers match on the slide, frame and front of barrel. If it is mismatched there will be no collector value.
If you could copy down all the writing and symbol, I am sure alot of people here could help with identifying its origin.