I'm assuming you are asking about credits used on ArcGIS Online ?? You may want to start by looking at some of the information here ArcGIS Online | Service Credits
If you are wondering how many credits you have left, login to you account. Go to the My Organization page, and # of credits will be listed in upper right.
And on that same page, if you go to
you can get more information about how your credits are being used.
There are also other location like Admin Tools for ArcGIS Online & Portal that might come in handy
If you are not talking about ArcGIS online or Portal...my apologies. I'm not sure what/how credits are used by the REST API in other uses.