Climate Hawks Votes is running a primary in which you can chose either Hillary Clinton, Bernie Sanders, or No Endorsement.
The web page where you can vote is here. You are required to enter some identifying information in order to eliminate or significantly reduced gaming of the poll, so the results should be reasonably fair.
There is a tendency for climate hawks (using the term generally, not in reference to this specific group) to favor Sanders on climate over Clinton, because Clinton is not 100% anti-fracking and anti-methane, while Sanders is. However, I think this is a bit unfair. Sanders has never been in the executive branch, and the Obama White House made its transition from being softer on climate change to stronger on climate change only recently. In fact, the two candidates for the Democratic Nomination are very similar in their stated positions on climate change. (See this post for more discussion on that, and links out to various other sources of information).
Also, though I like both Hillary and Bernie a lot, the truth is that neither of them really qualify as true Climate Hawks, in my opinion. Science has been telling us about the importance of climate change for years. By 1990, the reality and importance of climate change was clear, and should have permeated the political discourse during that decade, but it didn’t. Very few politicians can really be considered climate hawks by that standard.
More recently, a small number of politicians, none of whom have the names “Sanders” or “Clinton” have been pushing for implementing policies that will address climate change. They may be considered climate hawks for this reason.
In my view, the real contrast will be between whichever Democrat gets the nomination and, almost certainly (though with a brokered convention, who knows?) the Republican nominee. If you care about the climate, you will want to vote for the Democrat, whoever that is. Full disclosure, when I voted in the Climate Hawks primary, I went for “No Endorsement” for this reason. To my mind, this is in line with what Climate Hawks Votes has tended to do; They avoid giving endorsements to candidates without real climate-savvy records. But, the choice is up to you.