a good hint is what note it starts and ends on, but that's not a guarantee. You have to know what notes make up what scale, see which ones are in the song. If you have guitar pro, you can use the reverse scale finder to figure out the scale(s) used,
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Most rock songs are in E. Just try to figure out which scale the riffs (if there are any) resemble the most.
Start playing the major scale or minor pentatonic on a few frets, moving it around until it sounds right with the song. Alas! you have the key of the song. Thousands of songs are written with first position chords, so try those first and if those fail, move around.

You have to know a little bit of theory and develop your ear, but its all worth it. You can simply listen to a song and pick up the key and figure the song out.

After i taught myself how to do that, i never had to look at tabs again. Okay, maybe a little for the tough parts which i had no idea of what they were.