If you are giving the beer a sufficient amount of time in the primary, like a couple weeks, you get pretty good at just knowing that it is done. Now this wouldn't apply for a very big beer, or if you have something like certain Belgians, or bugs in the batch that are notorius for dragging out the ferment.
It's always good to check the OG when you think it is done, but there is usually no cause to be in there 4,5, or 6 times checking it. RDWHAHB and let the yeast do their work.
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