Not sure if this is standard practice for umpies, but when I was umpiring at first base, if there was a close play on a grounder, I would WATCH the bag and LISTEN for the catch. If the foot hits the bag and then you hear the catch he's safe. Logically, if it's the other way around the opposite applies.
I can't tell you how many umps just watch the close play as a whole and then try to make the call based on who they think should have made it or what it kinda looked like.
When I used this I always felt very confident about my calls and had no bones about telling coaches/players that they were flat out wrong if they decided to argue. I firmly believe that anyone who says "tie goes to the runner" has absolutley no clue because if you're paying enough attention you will notice that there is no such thing as a tie at first base.
Good luck and stay patient. It can be a thankless job but it can also be a lot of fun.