You need 3 things to hit VTEC
1) Correct Oil pressure - if you don't have enough oil, it won't engage.
2) Warmed up engine
3) You need to be loading the engine, won't work in neutral

If your VTEC isn't working, the car will feel gutless at high RPM, it will feel like It stops pulling as hard after 5000 rpm. You won't (or at least shouldn't) feel it engage, but you should notice that the engine pulls hard all the way to redline, I know my b16 does.