You will know when you lost your canopy. The computer will keep telling you this fact like every 5 seconds - it's really annoying. Also you will lose your holograms in the front of your ship because they project onto the glass. So if you are targeting a station and trying to FSD to it, you won't be able to see it in front of you, you will have to look -eyed at it and try to get it on a shard of glass at the edge of your broken window. Also there will be a blue timer in the top-right for the amount of oxygen you have left.