computing hash from a pkcs7 signature

From: Param (Param_at_discussions.microsoft.com)
Date: 06/29/05 

Date: Tue, 28 Jun 2005 23:45:01 -0700

Hi,
How to get the hash from a pkcs7 signature.
Is there any CryptoAPI for this.
What about the CryptMsgGetParam()?
If this is the correct API , then what I have to pass dwParamType?
Thanx in advance