A very great deal depends on where in the World you are located, and the type of equipment, and its application.
Usually the equipment itself needs to comply with some minimum design and minimum performance requirements. You yourself need not be licensed, but the testing authority that tests, and guarantees compliance must be.
Basically, you need to research the rules and standards governing the type of equipment in the places where it will be used.
By "place" I mean not only the Country, but the application, such as a domestic home, commercial office, heavy industry, aboard an aircraft, down a mine, in a hospital, or some other "place" of operation.
The rules and requirements differ greatly.
For example, if your gadget needs to work in a quiet environment, and the rules say it must emit no more than so many dbA of noise when measured in a very specific way, you get a registered acoustic lab to test it for you.
You yourself do not need a "licence" to do acoustic testing. You get someone else to test it for you. The sound lab will then give you a written document, proof that your gadget complies with the relevant standard sound test, whatever that is.
At the end of the whole process you can then sell your gadget and say it complies with the various recognized international standards. If challenged later, you can dig into your filing cabinet and pull out the results of all the tests to prove full compliance.
You yourself need no license, just good planning, organization, and first class documentation.