How to validate session token from Client

Hi,
I have created my own authentication service using MBaaS service and using it to authenticate my users into application. As specified in the document i am returning a json response along with the session token. I read in the client API $FH.AUTH documentation part that after authentication the session token will get added automatically in the header as X-FH-SESSIONTOKEN to all subsequent cloud calls. I am able to get this token in my request header but do not know how to validate it. Can you please let me know on how should i validate this session token in my cloud service?