I flied to Cyprus from Stockholm with Norwegian last week. On the plane it was written 737-800 or 737 Max. I wonder how you can tell them apart and if Norwegian uses the Max line!?
The planes are very similar (which is the whole selling point). That's why they can use the same safety instructions, which is probably where you saw the text "737-800/737 MAX8" printed. As others said, they are most easily told apart by the different look of their wing tips.
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.
However there are aftermarket wingtips for the 737NG (737-600/700/800/900) which are similarly split that a number of major operators have fitted.