Well, the size of the crowds that Trump draws versus the other candidates is a pretty good indicator that the polls showing him in the lead are correct. Indeed, I think they understate his lead, if anything.
This early in the election (remember, the FIRST primaries aren’t for another three months), the size of Trump’s rallies is more an indicator of the fervency of his supporters—and of his celebrity status—than of where he stands in the polls.