How can I tell how good a potential hire will actually be once on the job?

You really can't know how one will react, behave once they are actually on the job & dealing with the racism, bigotry, sexism, misogyny, lying, backstabbing, threats & attacks that go on in the workplace. Will they be able to ignore their coworkers' behavior, can they remain quiet about the hate speech, the couple folks who can't do their job & just cause problems with others (why we have such a turn over in the first place.) It's hard to tell, this is why we are constantly looking for new people because few will put up with the behavior of the couple "untouchables," that do & say whatever they wish without any repercussions & if their behavior, hate speech, threats, attacks are mentioned then to defend & cover for them, the person(s) attacked are threatened.

it's difficult. Sometimes the melancholy interviewee turns out to be a great hire.

There is no silver bullet. You have to trust your gut instincts to a large degree.

They won't seem "too good to be true".

If they have a good work ethic and drive to improve.

The person is a straightshooter and not someone with memorized answers and too good to be true.

You can’t. Trust your intuition

There is an old saying in HR that goes hire for attitude train for skills. Just be careful that you are not confusing likability with attitude.

If you can answer this, you can found the greatest worker placement service ever. This is the golden question. In my experience, the best way is to measure someone on cultural fit. That goes both ways, how will the existing team work with them, and what do they add. Sometimes a very different personality or background can take a team to a new level be adding a new perspective. Cultural fit tends to cover a lot of intangibles that are hard to cover in an interview. Most of the people who tend to match my teams culture and attitude also know when to dig in and stay late when needed, something you will never get a real answer on if you asked. Then finally, focus on the technical and what proof they have for their claims. Working on a team that did something big is very different than being the writer or the architect for a big project.

You can’t. All you can do is look into their eyes and try to understand who they are and why they want to work for you.

Think about what makes a good employee and ask questions tailored to that. For example, if preparation and presentation are key, ask “what did you do to prepare for this interview?” And “walk me through examples of projects you’ve completed in the past”. No hiring will be 100% as you expect from interviews, but the more you can get the interviewee to show examples of the key traits during the interview process, the more success you’ll have.

Are to they punctual? When they asked questions do they write down what is told to them and learn from them, or do they keep asking same question over and over?

Go out and have lunch with him. Do deliberately something wrong and see how he reacts!

The previous performance should be an good indicator as per what to expect, but again no sure way to predict performance on new gigs.

This is literally impossible to guage.

1. Ensure they're a good team and company personality fit. It doesn't matter how smart someone is if they can't work well with others, communicate effective, and drag down team or company morale. 2. Take the time to ask them direct technical questions in an interview or have them complete a task with access to the internet. (Searching online resources in a tech job is standard now. No one has to memorize it all but they should be competent using online resources to problem solve.)

Give them simple task that requires good work ethic and drive to finish in budgeted time.