A proclitic needs the following word to give it meaning since it can't stand alone. In 'qu'il' (that he), 'qu'' is proclitic and attached to 'il'.
Pro Tip 2/3
Pronunciation Flow
Proclitics often make sentences flow better by avoiding pauses between words. Say 'I've' instead of 'I have' to feel the smoother flow caused by the proclitic 'I've'.
Pro Tip 3/3
Detection
Identify proclitics by noticing if the word does not make sense by itself and relates to the next word. 'd'ans' in 'd'ans le parc' (in the park) shows 'd'' as a proclitic.