On the basis that specific skills and the level of expertise required are known then the following approach can be taken to determine the candidate’s suitability.
Firstly, through the interview process seek out the achievements and performance and through detailed questioning of these the extent of skills can often become apparent.
These skills can be technical hard skills and soft skills.
Where technical skills are important, highly specific and fundamental to the outcome of the role it is not unusual to test the candidate more thoroughly. There is a range of technical tests available from different organisations, however, you may consider your own work related tests. For example, this could be to analyse a piece of code or use technical scenarios and to solve specific problems.
For soft skills you may use behavioural style tests and personality traits or EQ tests. These are guides which highlight the consistency of the candidate's behaviours and their work and their greatest benefit is to determine what questions to ask the candidate and their referees.
Reference checking is an important part of the skills test process. Ask questions of not only the referee but also of other people the person may have worked with, for or managed in the past. By answering open-ended questions people paint a picture of the person's skills both technical and soft which either confirms the picture built up during the interview process or not.
One of the best measures will always be the opinion of ex-colleagues and staff. If they are not describing the skill, without being prompted, it is less likely that they have the skills.