Time Trial / Time Trialling / TT

Hard to tell without spending time in a lab estimating your aerobic threshold is in terms of output watts by monitoring lactic acid levels, blood sugar levels, oxygen consumption...

By "pull away from him for 15 seconds" do you mean you end up only losing 1:45?
Could be someone heavier but more aero getting an easy downhill ride to the turn but not having the output to beat you on the way back.

When was the last time you did a TT and actually hit your limit before the last mile? Do that, then ease off each week until you hit it with 10 metres to go.

Not exactly difficult. Next race, go out harder than you normally do and see if you can hold it. Either you will or you won't. No need for testing - the race is the test. It's only a 10.. not like you only get one shot at it per year.. :)

As for the heavy guy, more aero thing, I often have people pass me on a hill only for me to pass them again on the descent or flat. Gets annoying some times as I have to put in more effort than I'd want in order to keep away.