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

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

Sample Responses:

SOAP:

<soapenv:Header>
<LimitInfoHeader>
<limitInfo>
<current>45</current>
<limit>5000</limit>
<type>API REQUESTS</type>
</limitInfo>
</LimitInfoHeader>
</soapenv:Header>

REST:

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