You didn't ask me, but the answer is her policies, comments, and decisions.
https://www.theguardian.com/commentisfree/2016/jun/27/hillary-clinton-necono-republican-endorsements-donald-trump-policy-issues
At my count, this article only covers two indicators that clinton is a neocon, but they are two clearly-evidenced indicators.
Excerpt:
Several neoconservatives have spent years gushing about Clinton’s penchant for supporting basically every foreign war or military escalation in the last decade, including Kagan, who said in 2014: “I feel comfortable with her on foreign policy ... If she pursues a policy which we think she will pursue, it’s something that might have been called neocon, but clearly her supporters are not going to call it that; they are going to call it something else.”
Her campaign hasn’t really deviated from that position, either. While she’s hit Trump for being too erratic and dangerous a man to have in charge of the nuclear codes, she also promised more ground troops in the fight against Isis, expressed support for a no-fly zone in Syria (effectively a declaration of war against Assad) and called for more weapons for various rebels in the region.
Just this past weekend, we learned yet another lesson about what constant military intervention in the Middle East has gotten us: one more disaster where untold numbers of US guns and weapons fell into the hands of the people we are fighting. The New York Times reported that the classified CIA program that armed and trained Syrian rebels directly fighting Assad – a policy Clinton pushed for while in the Obama administration and that she has subsequently said we should expand – led to the systematic stealing of millions of dollars of US weapons, which were then sold on the black market and even contributed to the killing of Americans.
On the economic front, Hank Paulson – former Goldman Sachs chief and the man who oversaw the financial collapse and economic bailout as George W Bush’s treasury secretary – put himself firmly in the #NeverTrump camp while also endorsing Clinton in a Washington Post op-ed. After talking about Trump’s business acumen (or lack thereof) and Trump’s appeal to ignorance, Paulson says this: “I find it particularly appalling that Trump, a businessman, tells us he won’t touch social security, Medicare and Medicaid.”
Really? There are many “particularly appalling” things about Trump: that he congratulated himself upon the Orlando terrorist attack, has called for a ban on all Muslim immigrants, that he insinuated Barack Obama was working with Isis, and so on. But pledging to keep economic and health programs that tens of millions of Americans rely on is not one of them.
To be fair, Clinton has strengthened her wishy-washy language about protecting Social Security somewhat since early in her campaign (after being pressed on it by Bernie Sanders during the Democratic debates). But with the Clinton camp planning on pushing for more Republican endorsements, it does leave open the question: if Paulson is so appalled by Trump’s alleged positions on Medicare and social security, does he think Clinton is more likely to “reform” – ie cut – them too?
In any case, the fact that Republicans may have ulterior motivations for endorsing Clinton, even as her campaign pushes for more of them, is a prime reason why progressives should not abandon criticizing Clinton when they disagree with her. Holding her accountable for her positions in the general election is vital, even while everyone also loudly denounces Trump.
