They should research their elected officials before buying into the choices given to them by the media. Beyond that, John Q. Public should have no input into foreign affairs. That would be way too cumbersome.
As for how to research the candidates, I suggest visiting each of their websites and seeing what they stand for. Take a look at what they say they want to do and run it through the mental filter of reality. Consider if their goals are realistic and what the impact to the country would be.
Take the idea of Medicare for All as an example. Lots of enthusiasm as the candidates all say it is a great idea. But the financial cost of such a venture is estimated to be $32T over 10 years. That averages out to be $3.2T per year. Our GDP in this country is only around $3.6T/yr. So 89% of all money the government takes in would have to pay for this one program. That seems unrealistic and it would mean that other services would have to be cut drastically OR taxes would have to go up significantly across the board. Significantly like 100% higher than they are.
https://www.thebalance.com/current-u-s-federal-government-tax-revenue-3305762
https://www.forbes.com/sites/sallypipes/2018/07/09/choking-on-the-cost-of-medicare-for-all/#428185b56f3c
And I think people need to realize that there is no program or goal that will make everyone happy. So I, personally, like to look at the candidates that aren’t afraid to tell me bad news. They seem more honest to me.
Another litmus test I have used is one that can only be used on incumbents. Write them an email…ask them a question. I have found 3 reactions to this. The first is that I get nothing back in return. The second is that I get a form letter back, thanking me for my interest…but nothing else. The third generally starts like the second…a form letter…but is shortly followed up by an actual letter back from that representative’s office addressing my specific letter. That third one tells me that person is still engaged in how his/her constituents are feeling. These people I tend to favor at the ballot box as well, though it their values and goals are drastically off from mine that impacts as well.