Special:GadgetUsage is our friend.

Plus, mwgrep on wikis in the javascript namespace.

Gadgets and User Scripts on enwp

• most gadgets: don't necessarily inspect HTML -- 30% maybe use actual HTML most things might look for ids ...
• taking an inventory of gadgets
• no reliable way of knowing how user scripts ...
• a few gadgets everyone uses: popup, hotcat, (5 or so) ... ppl post in village pump in < 30 mins if broken
• some 20 or 30 that a few more ppl use and will take a while
• last category: used for specialized processes .. about 100
• page lists a lot of them