1) Watch for local inventory. If inventory has a trend to go down, a good sign.
2) Watch for ease of sales and speed of sell. If they start to sell more quickly, a good sign.
3) Talk to real estate agents to get their feeling. Are they happy? If you see smiles on their face, a good sign.
4) Of course, you can look at all the reports on the web looking for clues. Identifying a new boom trend should not be very hard.
5) Allow yourself at least 6 month or a year to determine the trend, just to ensure that the trend is certain.
6) I would also personally visit new home builders's sales offices, see if there are long waiting lists, lottery plan or any lines to buy houses. I will talk to salesmen and women to gauge their sentiments...

Where to flip? Flip new constructions and pre-constructions, they are the easiest and with least risks.