Short answer: Have a specific plan for marketing the house AND tracking the results AND a scheduled price decrease if the traffic is there but there are still no takers.
Long answer: They should be able to show some usage stats for their past advertising. (ie # of views they usually get on xyz sites, what type of membership status they have, the # of showings they get,, # of phone calls fielded, # of email inquiries received, # of showings, market trends, new listings, comparable homes soon to close, and agent/buyer feedback). If there are no stats for showing the success (MLS entry is my one exlusion) then its not worth doing in my opinion. The stats are usually very telling as to where the problem is (exposure, access, condition, or pricing) and what needs to be corrected to sell.
There's an infinite amount of combinations as to where the house should or could be marketed, most sources having very little credible evidence of success/failure. Trust me, there's a lot more sizzle out there than actual steak. Lots of talk about "coverage" and "exposure" and "circulation" with really sketchy evidence for results. We know, we use, have used, or looked at almost all of them. Even those we've proven to work, like certain listing portals, have some tweaks and turns that must be made to really make them highly effective.
Its a very technical discussion for me to get into EXACTLY what works for marketing and how it has to be setup to work best. Just make sure that whoever you list with will provide some meaningful stats to follow. Then watch the numbers and take appropriate measure when something is out of whack (like reducing the price when you have tons of showings, great feedback, thousands of page views, but no offers).
Either way its a great question, one that I wish more sellers would ask.