There are usually lines on the fermenter, secondary vessel and/or the bottling bucket that will show you how much you have. If you have a carboy for a secondary, you can simply use a sharpy and mark 1/2 and gallon increments on the carboy so you can see what's there.
I find that after a month in primary, putting 5 gallons in the fermenter nets me 5 gallons in the primary. The trub layer compacts as the yeast falls out over the month so much, that you get a near wet concrete like cake on the bottom and ALL the 5 gallons of beer sitting on top.
Some folks put 5.5 gallons into a primary to over compensate for trub loss, but I find that with extended primaries, or cold crashing AND careful racking that I have NO or little if any trub loss.
But the thing to realize is that, even if you have some trub loss, it's really NOT going to over carb your beer. If you look at carbing to style charts, you will see that every beer style has a "range of carbonation" and that the 1 oz/gallon that we use (4.5-5 ounces of corn sugar) produces between 2 and 2.5 volumes of co2, which is the average middle of the range for nearly all styles of beers.
So if it works out the you have that "extra" quarter or half ounce of sugar because you only managed to rack 4.75 gallons of beer into the bottling bucket, it isn't going to be that big of a deal.....In fact most Americans, having grown up with fizzy yellow beers, actually prefers
their beer slightly higher carbed than let's say the British, and we tend to be a little uncomfortable with beers that are carbed lower on the range, even with beers that are meant to be less carbonated. So more than likely you wouldn't notice it anyway.
Don't over think this....Relax.