Your effort will be greatly hindered by the Dunning-Kruger effect. This theory of psychology says that:
- Incompetent people tend to overestimate their own skill
- Competent people tend to underestimate their own skill
- Incompetent people fail at accurately estimating the skill of others
That means just asking people where they see their strengths and weaknesses will not give you any meaningful results, because people are notoriously bad at self-estimation of their skills.
Making an exam or interview to test the skill of people requires that the one who prepares that test is actually highly-skilled themself in that skill. Otherwise their questions and their rating of the answers will not accurately judge the skill of the people who are tested. Also keep in mind that such an exam will likely be rather unwelcome by your workforce: They are long out of school and university and likely feel that they grown up from the age where having such tests is expected from them.
So what can you do instead?
One way would be to look just at formal qualifications.
- What level of education do they have
- What certifications do they have
- Which training did they receive
- How many years of experience do they have with different technologies
This might give you a good estimate, but could still paint a wrong picture of certain people. There are people who are highly skilled in their field and never saw an university from inside, taught everything themself without having any official training and never bothered to get certified because they were too busy with getting real work done. On the other hand there are people who visited tons of training courses, somehow cheated their way through various certifications, have decades of experience and still don't know what they are doing.
Another more accurate way is to measure peoples skill is by measuring their success through benchmarking. Measure how successful they are at different projects and how productive they are at certain tasks. Important criteria for benchmarking are:
- Total time taken
- Resources used
- Number and criticality of defects
- Customer satisfaction
Then compare these numbers to those of similar projects and tasks at other companies. Depending on what your company is actually doing, accurately measuring performance in a meaningful way can be quite challenging. But it's the only way to objectively measure skill. There are business consulting companies which specialize in this kind of benchmarking. The advantage of hiring an external company is that they have access to the numbers of all other companies where they did the same and can compare you to them to give you a good estimate how well you do compared to others in the same business.