What are the dough strength indicators if mixing by hand?

I'd strengthen the dough a bit before adding the olive oil, perhaps 3-5 minutes with slap/fold until the dough starts to smooth out and hold its structure on the counter. Then, return the dough to the mixing bowl and mix in the olive oil by hand. The dough will break apart initially but continue to fold it over itself until the OO is all absorbed. It's ok that the dough doesn't smooth completely after adding the OO; it'll come together and show more strength during bulk fermentation.