Evidence of Stock Market Top?

Tech, Biotech, and Social Media stocks have been seeing much higher stock prices while the overall stock market attempts to set a new high. Companies like Facebook and Apple are building lavish new corporate headquarters, while Google's plans have been denied by the Mountain View city council. Such building behavior is often associated with "tops" in stock markets so we could be looking at a "correction" in stocks (down 20%) six months or so from now.