Another word for that feeling is invincibility tilt. I'm terribly results oriented and it is something I continue to work on - the basic idea is this:

In a vacuum you sit at a table and try to make the best decisions every time considering everything you know about the table and your opponent. There are two elements to this - the strictly mathematical facts and probabilities of the game and knowing your opponent and knowing how he is prone to act and react. What should not enter into any decision making process is whether you are in pain, enjoyed your lunch, had a fight with a friend or partner, whether you are up or down in your session etc - because then you make decisions based on what you want to happen rather than what is likely to happen. These are now no longer your best decisions.

In terms of catching weariness - like many things at the poker table this is something you catch on review and formulate a plan for which you then carry out in your next session.

When you do your session reviews pay special attention to the later hands played and try to identify the point at which you start making stupid decisions - make a note. Let's call it 100 minutes one day. Next day it may be 80 minutes and the one after that you do well for a solid 3 hours. First off this will teach you how long you can expect to be making solid decisions - then what you do is plan a session of a length that is short enough that you should be making only good decisions - plan in a break after that, but don't just spend the break at the computer looking at the tables waiting to be dealt in again - do some type of activity that is going to recharge your ability to concentrate (such as physical exercise, or meditation if that works for you).

And read some more of Spoon's threads on related topics (work ethics, lengthening sessions without losing focus etc).