Is there an API method to return how much API calls were used and how much total calls is allowed by my SF org?
Go to Setup | Quick Find System Overview and there you will find the API REQUESTS, LAST 24 HOURS. This shows you how many API calls you've made in the last 24 hours including today. For example, if you are viewing this on Monday at 2:30 PM, it'll show you the calls made since Sunday at 2:30 PM.
The "API Usage last 7 days" report
NOTE: 'View Setup and Configuration' permission on your profile is required to see this report.
The 'API Usage last 7 days' report shows you the calls made by user and date for the last 7 days including today. Take into account that the counters for this report reset at 12am Greenwich Mean Time (GMT) or UTC.
For more granular information, Event logging can be used. Refer to our SOAP API Developer Guide for more information.
Use Limits from REST API
Use Event Monitoring to check API Usage