During my adventures in boot fitting, I was told by a Riedell tech that their standard boots are one size below the ball. So a B ball will have an A heel. From what I know (and please, someone correct me if I'm wrong) that most off-rack skates have a B-ball, which is ladies' standard width. So if you bought off-shelf, you're probably in a B with an A heel. Men's standard is a D ball. So D ball, C heel.
Questions to ask yourself:
Are you in a standard (off-shelf) boot? Does the heel feel loose? Does it slip up and down or from side to side? Are you supported? Are you lacing tight enough? Are you lacing so tight because your heels don't stay in otherwise? Are you using padding to help prevent slippage? Do you have pain? Are you comfortable? Are you breaking down your boots? Have they broken down too fast because of my heel/support issues? Basically, how well does the skate fit you?
I managed to figure out my measurements because I was already in a 3A heel by sheer happenstance (it was my ball size that needed changing); it is still slightly too big. I pad out with Bunga pads and do a tighter lace. Also, I checked it on a measuring tool before purchasing new boots.
That might actually be a pretty good option - find a good shoe store with properly trained staff and have them measure your feet. They can give you the measurements (in inches, whatever you need) and you can compare them to boot size charts. Caution - the charts I've seen generally don't cover heel sizes. Also, don't hesitate to call skate manufacturers. They want to talk to their customers (and make a sale).