tl;dr on the MAX the wing tips split into two (above and below the main wing), on all previous 737s, the wingtips only curve up. Also the engine cowling has some cool looky jaggies on the trailing edge.
I had not seen the split winglet on non-MAX 737s. I stand corrected. I did mean the CFM engine as a distinguisher between 737 models, not as a identifying feature for 737 MAX vs all other 2 engine smallish jets.
There are two features you can look for:
https://aviation.stackexchange.com/questions/48929/is-this-a...
tl;dr on the MAX the wing tips split into two (above and below the main wing), on all previous 737s, the wingtips only curve up. Also the engine cowling has some cool looky jaggies on the trailing edge.