Panting is a really good sign you are over your AT. However it does not happen until you have been over your AT for a while and you have incurred a substantive oxygen debt. The ideal is to stop before you get that far. This is a hardwired physiological response, and so I think its fairly reliable. However as I said you have to stop before you get that far. Once you start panting, breathing heavily or have distressed breathing the damage has started.