^^ I think this can be REALLY hard to learn, because it requires a lot of experience that has very little to do with pedalling.
1. Am I strong enough to do this and be ready for an attack later? (or) Do I want to let him work for a while longer?
2. Am I strong enough to do this if the bunch wants to chase later? Is he strong enough to stay away? Can you beat him in a sprint?
3. Are the other riders interested in letting him go? Are the front runners capable of bridging the gap?
My first B grade win came with a bunch of unfamiliar faces who spent a bit of time at the front... I think they were a bit green, and they weren't capable of launching a chase when 2 guys took off, and certainly weren't capable of launching a chase when I charged off the front to bridge the 300m gap.
I don't know how much of that can be taught or learnt - some people just have an awareness of the posture on the bike, the breathing, the difference between someone suffering and someone pretending or just hiding.
I don't know how much is ever a gamble - it's an informed decision. But you're totally right - sometimes you aren't sure. Other times you are.
I would totally recommend eyeballing the whole field on the road if you get the chance to see the difference. I have jumped off the front at the Penrith Regatta Centre, and if you allow the bunch to pass you after a lap on the finish straight, you can safely check everyone out. Some guys will be redlined just holding position in the bunch, others are playing doggo, others are simply riding. You can see it, but it's often quite hard to do this - I wouldn't feel comfortable doing this at Lansdowne on the flip flop.