This article will detail how to check your recent API calls/usage.
You may receive this notice from Salesforce if you have reached your API call limit. You can look into this by checking a report:
1.) Any user can see the number of API requests sent in the last 24 hours.
- Navigate to Setup and enter Company Information into the Quick Find box
- Click on Company Information
- Look for "API Requests, Last 24 Hours"
2.) If a user has the “Modify All Data” permission, the user can view a report of the API requests sent for the last seven days.
To view the information, click the Reports tab, scroll to the Administrative Reports section and click 'API Usage Last 7 Days.' Users can sort the report by any of the fields listed in the Summarize Information by: drop-down list.
Note: This Report Type is only available in Classic.
There's not really a place that shows what the specific call outs are, but the top suggestion on this help article might be a good suggestion:
Generally speaking, it's guess and check as far as figuring out what's eating up your API requests. The reason you're seeing < 100% usage could be that most of the API requests happened around 24 hours ago and have since been removed from that report, since it's a rolling 24 hour window. Check your login history as well (System->Manage Users->Login History), and see if you notice integration users logging in or users/developers logging in through the API (for example using eclipse or mavensmate development evironments).
Between the login history and the system overview mentioned in the link above, you should be able to narrow it down to the culprit.
More information: https://developer.salesforce.com/forums/?id=906F0000000D7nAIAS