Okay, when I first started brewing I had a very hard time knowing how much beer was fermenting in my carboy and when I started doing all-grain I had a very difficult time knowing how much wort I had in my kettle before I transferred to my carboy. Why does this matter? Well, if you have no idea how much you have fermenting and you don't keg your beer, then how do you know how to properly batch-prime your beer? (trust me this is important, if you don't think it is then wait until you have a hiroshima bottle bomb go off in your house). This is just one example of course. I have some tips you guys.
-Have a carboy? I have a pitcher I bought from the dollar store that measures up to 1 gallon. Pour on gallon in and mark it with a sharpie. Pour another gallon and mark it with a sharpie. This will let you know how much beer you have in your fermenter.
-When you are boiling your wort it may be necessary to see how much you have in your kettle. If you don't have a fancy kettle that lets you see this information, then take whatever stirring-implement you use and make it a measuring stick as well! take your kettle, fill it with one gallon of water and mark it (I use a wooden spoon so it's easy-I carve a notch in it). Mark the gallons on your spoon so that when you touch it to the bottom you can see how much your boiling. And if you have a desired post-boil volume and you start to fall below it, add some water during the boil to get up to where you want to be (this ensures that the water you add also ends up being boiled)
This may seem like pointless information but once I figured this out it helped me a ton!