I've found a strategically offered twenty "for a quick look"(try charm first) helps to get in an occupied house. Remind them you may buy the house, they may want to get on your good side. A good entree is to ask, "I heard this house is for sale...?" offered tentatively and as gently as possible. I've seen several foreclosures recently (and bought two) for nothing, the people seemed happy to show it. You can't really do a full on inspection, but remember the bank did that before they loaned on the property and it passed then.
Even rented the house back to one gal; she paid me $1000 (rent & dep.) the day I bought it--for half of market value--last week. She was mowing the lawn the day before the sale. If that's not a good sign of a good tenant, then I'm a dupe. She was unemployed awhile and got too far behind in payments, but seems stable now. I know it's not good practice to rent back, but it felt good this time, and she sure was happy about not having to uproot her family.