They inspect deep drains and foul water. Beis Shammai say they also inspect garbage dumps and loose soil. Beis Hillel say that anyplace a pig or a weasel can go doesn’t require inspection.
A porch isn’t considered a non-Jewish dwelling. Rabban Shimon ben Gamliel says that a non-Jewish city that has been destroyed isn’t considered a non-Jewish dwelling. Eastern and Western Caesarea are graveyards; Eastern Akko was in doubt but the Sages ruled it ritually clean. Rebbi and his court voted about Keni and ruled it ritually clean.