Yes, there is a way to tell during veg, but not until it is a mature, well developed plant. I'm guessing, around 50-60days into vegging. The plant will have pre-mature hairs showing. I, on the other hand, find it very hard to decifer between a male and female pre-hairs, so I always wait until the first 2 weeks of flowering are over before I know for sure.

Yes, if you take the males out before they blow their loads, then your other plants will be safe. You don't have to worry though, the male plant has to be very developed before it spreads its pollen. At that time you will know for sure whether it's a male or female.