You can install cables and clamp them in a vise or have someone hold onto the ends whilst you click through the stops. A quicker way to see how they function is to simply run through the clicks. If you find one of them is sticky, try dousing the internals with WD-40 or other similar product. I have miraculously resurrected shifters using said method as have many, many users of this forum.
John
PS If you search for gummed up shifters or other similar terms, you will find lots of previous posts on the subject.