How to determine "font capabilities", possible font-weight values, etc.?

Question

Web development

Tell me there is a file with the font "Clarendon". Tell me, how can I determine what properties are available to it?
For example font-weight: 100, 200, 300 are the same. 400, 500 - a little more, but the same among themselves ... That
is, apparently 3 variations of thickness are installed. Then, how to determine what can be used (except by eye)?