If you mean the buff itself, it has a number on it that shows you how many stacks you have. That number counts down when you use a weapon skill that benefits from it, such as Hot Shot or Split Shot.

If you use Reload that gives you 5 stacks by default so your buff says "V". When you fire off a shot it goes to "IV". You should only use Quick Reload if you're empty or have IV or less. Using it at V wastes it.

There is a more detailed guide available, too!