This is like the million dollar question on everyone's mind. In short i can say that no one in the market will know if were at the top or bottom till a few days after it makes the move. About PE being high we aren't in the bubble territory yet and we have gone much higher in the past.
For trading if we look at fundamentals and analyse then the wel be wrong more often than not. In trading its better to follow price. Incase your investing then PE ratios etc make sense trying to track and buy at lower levels and sell higher. So in that sense we maybe overbought as of now in investing terms. But for a trading perspective nifty can go anywhere.