the performance(rebound quality seems to have deminished) and the colour of the bat usually is much darker than usual and the bat sounds different a more thud than ping.saying that though i never oil any of my bats ever i
if the wood goes darker/softer than you would normally expect or if it begins to pick up heavy then you may have over oiled it
One at the start of life is enough, any more and you haven't used the bat enough as it should be smashed up from all the runs you score at the end of the season.
You do not need to oil if using a scuff sheet, they seal the face better than oil. Plus oil will make it harder to stick down.