Basically the sooner you figure out they're going for HC the better. If you see they have 4(+) military labs they are definately going HC... even 3 military labs is a fair warning.
Advent: Repulse and mass illum. Bombers are OK if they got a marza, but if they get a Kol you'll need a lot of bombers.
Vasari: try and sneak an orkulus since they'll be skimping on fleet to get HC. That or kill their Kol with assailants, then set loose bombers.
TEC: pretty much forced to scout early and get bombers.
If you are close to them (2ish jumps) you can sometimes LRM rush them and take out labs before HC gets up, though sometimes HC rushers open with scouts.