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I suspect the answer is read the source, but

A) how do I know what bits I will not be using B) how do the JS and LeSS files interact (can I leave out the JS as easily as the less?)

It's done intelligently, so you don't use tables, then you leave out tables.less. If you don't want tabs, then you leave out tabs.js.

Also the chrome dev tools has an "Audits" tab which will reveal unused CSS rules. It's not perfect, but it gives you a good idea.