In WoW, there are plenty of abilities such as blood boil that have area of effect. This area is usually measured and denoted by yards. A yard is easy to estimate in real life, but how is it estimated in the game? If a spell has a radius of 10 yards, how far does it really stretch in respect to my character?
In general the easiest way of finding a giving range is to see what spells you can cast on your target and moving back and forth to see them be available or not, just remember that the target needs to be targetable by the spell you look at (hostile target for damage/debuff and friendly for heal/buff), I don't remember if the default skillbar supports this but some skillbars will grey out abilities when you are out of range of your target to make it easier to spot.
Another easy way of determining range for some ranges is using abilities where you have to place a casting circle. A Death Knight can use Death and Decay to spot a 10/20 yard range(10 yard from center to outer circle or 20 yard from one side to the other), for your example you can start the DnD move the courser over your char and the placement circle now shows you a 10yard range that you know your Blood Boil will hit.
The last thing I know you can do is get an add on like Range Display that will try and guess the range to your target.