The trick is to track inbound contacts on the various platforms so that you know that they originated from the specific ad being run.  If your call to action is to have a prospect call you, have a dedicated phone number and track the calls.  If it’s to go to your website, have a dedicated landing page (or maybe even a dedicated URL) that corresponds to that ad.  If it’s to have them mail you something (?!?), have a dedicated address (maybe a PO box or an ATTN line) so that it, too can be tracked.  Then collocate all of the data to determine you ROI and see if the print ads are working for your target demographics and adjust as needed.  If you’re running an A/B ad you may need multiples of the above, but make sure everything is tracked so that you have an accurate ROI.