A hex editor knows nothing beyond the hex the file contains, and maybe some ways it has been programmed to show said data (look down the bottom of the screen for the better ones and they will probably tell you what that value is if it is signed, a float, a DOS style date....).
ROM hackers however have hex editors made specially to take table files which are files that contain lists of what hex characters correspond to what decodings, some normal hex editors have limited capability here but you will want a hacking focused one eventually http://www.romhacking.net/forum/index.php/topic,21323.0.html
The main reason for the hacking focused ones are although some conventional editors will have basic encoding support then I have not seen one that easily mixes 8 and 16 bit encodings, allows for variable length decodings (see dual tile or multi tile encodings -- one value comes back as more than one character) and even wildcards, new lines and variable names can be tricky in them.
Same across games? Hah -- it can be different within a game, even within a passage, though such things are rare and for the most part you will probably only be finding one encoding if you are not hacking NES games, though I think Phoenix Wright might be one of those and menus can often be different to the game proper. That said there are encodings that some games share with the rest of the world (many DS games will use a least part of the shiftJIS or EUC-jp encodings), with other things from the dev (Capcom did this on the DS if memory serves) and for megaman ZX then I did that as an example hack in my GBA/DS docs http://www.romhacking.net/forum/index.php/topic,14708.0.html
Automate it? For languages with a set alphabet and order in it (most European languages) then there is a powerful tool called relative search. For languages without it (so Japanese which has no real order for anything, though some mildly enforced orders for the kana at times) then the only way you are not spending time is if the order is copied from something else (shiftJIS and eucjp were already mentioned and popular choices, other games also happen here) or is determined mathematically/logically (some games might use frequency of appearance, order in the script or something like that).
Even with a table the editor is not going to know what is text, it will just blindly decode like it always does.
Anyway all this is covered in the introductions to text hacking so I will leave it there. http://www.romhacking.net/start/