Check if SPUser has access to an SPWeb object without causing an Access Denied redirect

You can try:

The out-of-the-box method DoesUserHavePermissions() on an SPWeb object does not take indirect membership into account (i.e. the current user is a member of a domain group that has access the the SPWeb being checked). To get around this, I use the following method

DoesUserHavePermssionsToWeb(SPUser, SPWeb)):

private bool DoesUserHavePermssionsToWeb(ref SPUser user, ref SPWeb web)
{    
   bool hasPermission = false;     
   SPBasePermissions perms = this.GetPermissionsForUser(ref user, ref web);    
   if (perms.ToString().Contains(SPBasePermissions.Open.ToString()) || perms.ToString().Contains(SPBasePermissions.FullMask.ToString()))        
      hasPermission = true;    

   if (!hasPermission)    
   {        

      // Check the users groups - this is for indirect membership;        
      foreach (string groupLoginName in this.GetCurrentUserADGroups()) 
      {            
         try            
         {                
              SPUser groupUser = web.SiteUsers[groupLoginName];                 
              perms = this.GetPermissionsForUser(ref groupUser, ref web);                
              if (perms.ToString().Contains(SPBasePermissions.Open.ToString()) || perms.ToString().Contains(SPBasePermissions.FullMask.ToString()))                
              {                    
                   hasPermission = true;                    
                   break;                
              }            
          }            
          catch { }        
      }    
  }     
  return hasPermission;
} 

private SPBasePermissions GetPermissionsForUser(ref SPUser user, ref SPWeb web)
{    
     SPBasePermissions perms = SPBasePermissions.EmptyMask;     
     try    
     {        
         SPUserToken userToken = user.UserToken;        
         System.Reflection.MethodInfo getPermissions = typeof(Microsoft.SharePoint.Utilities.SPUtility).GetMethod("GetPermissions",            
         System.Reflection.BindingFlags.NonPublic |            
         System.Reflection.BindingFlags.Public |            
         System.Reflection.BindingFlags.Instance |            
         System.Reflection.BindingFlags.InvokeMethod |            
         System.Reflection.BindingFlags.Static);         
         perms = (SPBasePermissions)getPermissions.Invoke(null, new object[] { userToken, web });    
}    
catch { }     
return perms;
} 

private System.Collections.ArrayList GetCurrentUserADGroups()
{    
       // Get the current groups for the logged in user;    
       System.Collections.ArrayList groups = new System.Collections.ArrayList();    
       foreach (System.Security.Principal.IdentityReference group in System.Web.HttpContext.Current.Request.LogonUserIdentity.Groups)    
       {        
            groups.Add(group.Translate(typeof (System.Security.Principal.NTAccount)).ToString());    
       }    
       return groups;
}

taken from here:

http://www.mylifeinaminute.com/2008/11/21/custom-method-for-checking-permissions-on-a-spweb-object/

hope it helps :)