As above stated, the only way you'll know it's at FG is if you have a pair of matching readings 3+ days apart. OR, just give it enough time to finish, clean up after itself, and then let you know it's ready for next steps.
Brew more batches, try using as few different yeast strains as possible so you can learn how they work at the fermenting temperatures. IMO, this is one of the best things you can do for your brewing. You'll find out which yeasts produce what flavors at what temperature ranges and such. I've been doing this for some time now, using the same few strains of yeast for my brews. I also know what temperature my brew is fermenting at (thermowell with a sensor down it). I give the batches enough time to become great and be ready to go to keg. I don't fuss with them, take extra readings, etc. I simply KNOW that the yeast will give me what I want as long as I've given it the time it needs (dictated by OG, pitching a proper yeast colony size, and temperatures).
I suggest reading up about making yeast starters (especially for liquid yeast strains) and fermentation temperature control. Those two items will do more for your batches than a lot of other things.