checking user Authentication while calling webmethods from javascript

hi all

In my Webapp i am using form authentication. 

one of my authenticated aspx pages has a static webmethod which is being called from client javascript. it is being called working as expecting till user session on live, once session expires calling webmethods throwing error stated that 'authetication failed'

for that i want to check if the user session is on live, so that i can stop calling webmethod after session expires. can any body tell me how to resolve this.

Thank u all

Sree 
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