What is the best way to ascertain the true potential of a candidate (for the required role/vacancy) while hiring?

I believe that while hiring a candidate we should not insist only on experience because sometimes we should give to new people chance to show us what potential they have the most important to look in it how much he is interested in the job and how much he needs a work.
Because for sure if he's very interested so he or she will do their best to succeed

i believe that personality,attitude and eagerness to learn are key factors.

after all you are looking for an employee that would fit or match the image  and qualification the concerned department manager is looking for.

always invite the HR manager,the concerned  department manager and an adminstrator to be present at the interview.

you will shrotlist the best candidate.

Instead of researching on his previous experience in the same roll, Just explore his past like his education and the failures and how did he overcome it and listen to the important life incidents and how did he deal with it. Ask about his hobbeis, interest. Ask about the most happeist day in his life and why,what will make him keep himself happy in his life.

After creating a friendly atmosphere, Let him be asked about his areas of expertise or the skills required to perform the job that he is beng interviewed etc. Almost all candidates lose confidence when they don't face a friendly atmosphere. Let him be brave and couragious to say the truth.

Interviews seldom give insight into the employee's full potential and abilities.
Lots of times, those who are skilled at giving a great interview are not the most capable or hardworking employees.
On the other hand, there are employees who may not have great interview skills but are excellent workers.
That's why it's best to test the employee by putting him in a role similar to what he/she will be doing and then see his/her work and abilities.
A short test period of like a week will provide insight into the employee's skills and work habits.

To my mind following should be the process for the hiring a candidate:1.
The company / their placement agent should place the job vacancy on the job portals.2.
The candidates should apply for the jobs after viewing the Job Description.
The above two things are already in place.
Currently these steps are leading to thousands of applications for one job posting.3.
Companies should look into the profile of the candidates.
They should check the details about education, residence country, age, marital status, strength / weakness highlighted in the resume etc.
At this stage, not much concentration should be given for the experience.4.
Each candidate should be sent one business case study for analysis and making the report.
The candidates should send their reports within7 working days.
The companies can also send a HR questionnaire.5.
With the process mentioned in point4 lot of casual candidates will walk away and companies will left only with serious candidates.6.
The replies received from the candidates should be analyzed properly along with their experience shown in the resume.7.
The shortlisted candidates should be called for interviews.
Though this process is little long but with this process the companies will be in position to read the candidate mind and their capability to handle the job.
On the other hand there will be limited number of applications selection.

its depend upon atmosphere, working style, and the main thing is candidate body language that is very important while hiring and i think other skills is automatically rise when he working.