http://msdn.microsoft.com/en-us/library/microsoft.sharepoint.sppermissioncollection.aspx

SPSite oSiteCollection = SPContext.Current.Site;
using(SPWeb oWebsite = oSiteCollection.AllWebs["Site_Name"])
{
    SPList oList = oWebsite.Lists["List_Name"];
    SPPermissionCollection collPermissions = oList.Permissions;
    SPUserCollection collUsers = oWebsite.Users;

    foreach (SPUser oUser in collUsers)
    {

        foreach (SPPermission oPermission in collPermissions)
        {

            if (oUser.ID == oPermission.Member.ID)
            {
                 Response.Write("User: " +
                    SPEncode.HtmlEncode(oUser.Name) +
                    " Permissions: " +
                    oPermission.PermissionMask.ToString() +
                    "<BR>");
            }
        }
    }
}