...by the clock... is the most natural option. We also say 'by my watch' etc when we are looking at time.
All of these sentences suggest that perhaps it isn't actually 10.30 though, just that that particular clock says so, and it may be wrong. (otherwise why would you bother mentioning the clock. You would just say 'it is 10.30' if you knew the clock was right. By pointing out that specif