Using OrgLimit Class, we can check API Request Limit using Apex in Salesforce.

DailyApiRequests in OrgLimit class holds the Salesforce 24 hours API Usage.

Sample Code: to check Daily Usage

Map < String, System.OrgLimit > limitsMap = OrgLimits.getMap();  
System.OrgLimit apiRequestsLimit = limitsMap.get( 
    'DailyApiRequests' 
);  
System.debug( 
    'Limit Name: ' + 
    apiRequestsLimit.getName() 
);  
System.debug( 
    'Usage Value: ' + 
    apiRequestsLimit.getValue() 
);  
System.debug( 
    'Maximum Limit: ' + 
    apiRequestsLimit.getLimit() 
);  

Output: