It may seal just fine at idle because there isn't enough pressure in the bottom-end to push the oil & coolant into your combustion chamber. Once you get it going then it builds up that pressure and then the leaking begins.

The tell-tale signs of blown head gasket are white smoke out of the exhaust and oil in your coolant. You can also look for bubbling around where the head meets the block, but if the break is on the inner portion of the head gasket then you might not see the bubbling. A compression test might also reveal a head gasket problem.

If I were you, I'd replace it. I'd do the other things with it as well: timing belt, tensioner, intake/exhaust gaskets, etc. But, that's the way I am.