If radar is always on, they'll spot you immediatly. If it's on fire only, they'll see a red spot on their radar. No radar is the bast for saboteurs.
I now spot them quite easily, because they always come from odd directions. I mean, a supposedly friendly guy coming towards HQ 1min after the start of the game, that's a bit suspicious.
Other interesting stuff:
- As you mentionned, an ISA sab disguised as a high won't have glowing red eyes (and vice versa, a high sab won't have the blue lights)
- If there are ennemy turrets or drones around not shooting at a "team mate", that's suspicious.
- When you walk across someone, point your crosshair on them, then get it off, and back on them again. I've noticed that when you do this, the displayed name above a sab's head will often change each time you aim him again.
- A sab disguises as a rifleman. These are pretty rare now so when you see a rifleman, it's probably a sab.
- And of course, if you're quite close, your crosshair will turn red.