How we can check API limits already used in any organization by using REST or SOAP API ?

SOAP API and REST API always returns header after making successful call to Salesforce.

Sample Responses:

SOAP :

1	<soapenv:Header>

2	<LimitInfoHeader>

3	<limitInfo>

4	<current>45</current>

5	<limit>5000</limit>

6	<type>API REQUESTS</type>

7	</limitInfo>

8	</LimitInfoHeader>

9	</soapenv:Header>

REST :

1	Sforce-Limit-Info: api-usage=45/5000