Can anyone point me, How to validate access token when working with SAP Ariba APIs?
There is API to generate access token and refesh token but I didnt see any to validate one.
There is no separate method to validate an access token against the OAuth server. The only way to "validate" the access token is by calling the API that your credentials have access to.
Remember that you created an application in the developer portal and requested access to an API. The credentials for that application will only have access to the API that you requested access for. So, once you get an access token, that access token can be used to call the API. To "validate", just call a method of the API that your application has access.
For example, if your application has access to the Supplier Data with Pagination API, call the /vendorDataRequests method.