Security token lifetime

I have my own STS which derives from Microsoft.IdentityModel.SecurityTokenService class. I have a question about token life time.

I know how to set token life time. But I don't know what happens when token is expired.

For example, I issue token with life time of 2 seconds and it expires by the time it reaches RP, what happens?  WCF/WIF throws an exception or do I need to check it programmatically? If I need to check, How do I do that?

Can client know if token is expired?

Note: I am using WCF.

---

MP