Proper datasheets specify the pinout unambiguously, mentioning "tab" directly, or specifying "tab = pin #x".
But, when they fail at doing that, you just need to know that on such classical tab + 2 legs packages, the tab is always pin #2, and the legs are #1 and #3. Many such packages even have a short stub leg #2 available, you can see with a multimeter how it connects to the tab. I sometimes (ab)use this stub leg, for example solder a bypass capacitor there.
No, the tab or thermal pad isn't always ground. Often it is, and when it is, it's quite helpful.