How do you tell which type photo this is?

Smittys

dark on the back is shadows

Ironmanfan

impossible to tell based on that unfortunately; doesn't appear to be a Type 1

IMF

jay0791

Not an expert but I think type 1 and maybe 2 will have typewriter info on the back.
Blank back would lead me to believe its type 3

A photography shop could probably age it for you within a few years.

Nice photo

1951WheatiesPremium

Some information and guidelines from PSA...

https://www.psacard.com/services/photoprocess