Differences between 767-300s

How do you tell the difference between a 767-300 and a -300ER. I know there are some 767-300ERs that dont have winglets. Any info is appreciated.

Thanks,
bw50505

en.wikipedia.org/wiki/Boeing_767 is a good place to start, mainly just the increased MTOW and range due to extra fuel is the difference, the winglets are a retrofit.

No 767’s left the factory with winglets.

There’s not really any way to externally identify a 767-300 from a 767-300ER (other than looking up the registration).

Wikipedia makes an important note - only 81 -300s are still in service compared to 527 -300ER’s.