You have the same proof as you would get from any other person you don't know personally. I can tell you I'm a staunch Conservative with only Constitutionally sound values and principles but, you would never know for sure unless you personally knew me.
Hillary otoh has shown us every evil side she possesses.
And I bet if you were running for office you would have some way to show me you were that staunch conservative. What boards have you been on? Fundraisers have you headed up? Campaigns you've worked for? Legislation that you lobbied for? Organizations that you volunteered for? If you own a business what charities did you regularly support?
Trump is supposed to be some big name business man and all we hear about it how he gave to both sides of every issue, democrats and republicans in order to help his business. Is there any record of him hosting fundraisers for the National Right To Life Committee or Salvation Army or Samaritan's Purse or GOA or NRA or name your conservative political activist here, at any level or consistently over any number of years? As far as I can tell Trump is all about helping Trump and anything he does that I know about has some self-serving goal to it. Do you have info to the contrary?