Birdbox maintenance

We have a bird box in the garden, attached to the side of the garage.  There was some blue tit activity around it this Spring (we saw blue tits going in and out) but are unsure whether there was an actual nest or chicks in there.  Should we clean out the box at all and if so, when would be a good time to check and clean the box?  Obviously don't want to disturb any birds.  If there is a nest in there, should it be cleared out or left for next season? Thanks!