I would have thought that past 1,000,000 downloads you could trust the plugin but that is shown to be wrong. With Windows software I do a skim of the Wikipedia article for controversy as a sanity check but there aren't any for these extensions.

Previously I would search the apps name + some obvious terms like malware but those results are too spammy to be helpful now. Extensions are very useful, so I'd hope there'd be some reaction from Google on this.