As others have said, I would generally let the masseur set the pace.
If they place their bulge in my hand then it's an obvious green light to touch them but I try not to get in the way of the massage. I don't touch them when it's likely to interrupt their flow. Sometimes I move my hand towards a intimate part of their body but hold off from being pushy about it and see if they want to move away before exploring further.
It's like when the massage gets more sexual, I wouldn't try and suck or finger a masseur, for example, unless they make it very obvious they're open to that.
I had a masseur recently who got really into the sexual part and ended up riding me, but he made all the running, I'm not going any further unless explicitly invited to.