Just use a cloth, a bit of spirit, and wipe it clean. No need for laboratory conditions, once its dry just give it a wipe with a lint-free cloth. Obviously you don't want any grit in there, but its very shiny inside so you'll be able to see any muck quite easily.

Just tighten the sump bolts back up sensibly, bear in mind that there is a rubber gasket around the lip so I'd tighten it up in a staggered pattern, like you do with wheel bolts.