One of the biggest indicators is sudden change in volume and a majority of buy orders. But this can also be artificial, where miners suddenly change to a new coin.
Nope, to my experience, I have seen many coins where volume gets decreased in spite of increasing while they prepare to pump up a coin.
As well, when the price starts tickling too quick and you see movements in a coin which has not moved since a week or a month, then get in because it's a sign of a pump getting ready to shoot it up to previous highs or even more. But I would advice you to stay away from a coin which is already getting pumped and look for something better, because one can never figure out the levels at which they should leave it because the more higher it goes, the more greed it gets in our mind to wait more and there we take the wrong decision of stopping by. Remember, it's never too easy to get out once price starts slipping down.