If you see a broken vent with only one T in the cell, and you did not see them knife it, you cannot do anything. Maybe their cellmate escaped. Maybe they disconnected.
You cannot go into a teleporter unless you actually saw with your own two eyes that a T went in it.
You can pursue a broken vent if you saw nobody enter it, but have to actually pursue the vent.
On a normal day, you cannot under any circumstance break a vent as a CT, period.
You should be paying attention to your Ts at all times so I don't really know why you're complaining about cheating.
Remember that if you see someone go into a vent and you know where it leads, do not go to where it leads and camp it. You must persue the vent. If you can go through the vent without breaking it that means it's a T secret and you should not go into it unless you saw a T actually go in there.
Also last but not least when you see the colour or hub item of a rebelling T but not their name, you can't do shit about it. Maybe the rebeller has every colour bought and changed to the same colour as their cellmateI. I know I do and sometimes I do that to see how dumb the CTs are.
That's pretty much it for dealing with what you see vs what you gotta do.