How to identify 使動用法 in Classical Chinese?
the correct interpretation should be 使衆人誇贊[游者] (cause everyone to praise [the traveller]). 


imo, such interpretation is incorrect.


  越無車﹒有游者得車於晉楚之郊﹒輻朽而輪敗﹒輗折而轅毀﹒無所可用﹒然以其鄉之未嘗有也﹒舟載以歸而誇諸人﹒觀者聞其誇而信之﹒以為車固若是﹒效而為之者相屬


the text, in brief, should be interpreted as:

a wanderer (游者) brought [a broken carriage] by boat, back to his village (舟載以歸); and (而) exaggerate (誇) [the goodness of the carriage] to his villagers (諸人). onlookers (觀者) heard (聞) his exaggeration (其誇) and (而) believed it (信之)

classical / literary chinese is another paradigm. using grammar to analyse is, imo, unachievable. it’s like using newton physic to solve the “schrödinger's cat” case.

have fun :)