If guests haven't responded by two weeks before the wedding, call them and inquire if they're planning to attend. (There's a chance that they didn't receive their invitation or that their response card got lost in the mail.) If you attempt to contact them and can't, have a family member or the groom try again. If that doesn't work, have them leave a message essentially expressing your regret that they won't be able to make it, and thus you can safely presume they're not attending.