A bubble/ scam is often known only when its bursts out. But we can possibly look for some indicators to see whether we are in a bubble.
Trying to list such indicators here
1) When everyone believes something is a very good investment ( including the layman/uneducated/elite and all) and can never falter.
2) When most of your neighbours invest in something and cannot justify how the ( more than average) return can be made.
3) Newspaper /media focus shifts too much on the investment opportunity and you start feeling that you will miss the 'Bus' , if you do not get onto it now.
4) There is a strong justification for the thing to be a good investment and 90% of people agree to the justification . There's nothing unique about what you know about the investment.
We have seen stock market bubbles, teak plantation scams, Benefit fund ( NBFC) bubble and burst, IT bubble (recollecting all those that are significant and that I can remember ), oil spikes et all.
Two things that are falling into the category ( you need not agree with me ) as of now are real estate in India and to some extent gold...
But still if you want to invest in real estate , go for Dubai and not for Mumbai ...why ????? read this in ET :-)