It seems too time consuming to tune into each channel, make a note of those you can't see (wait for the message, click A, then tune next), then go to the guide's "skip list" feature and manually select those you can't tune in.
If you search for a program, and it is scheduled on a channel you're not subscribed to, Contour allows you to schedule it (even for series), tunes to that channel, then lists it to the "missed recordings". (!?)
Thanks