Is there an easy way to detect whether excessive foaming / gushing is the result of over carbonation or a gusher contamination?
Perhaps a telltale taste or other indicator?
If you took a specific gravity reading before you bottled and were confident that it was at final gravity, de-gas a sample and take another gravity reading now. If it's the same, it's over-carbonated. If it's noticeably lower, then some other wild yeast or bacteria else has likely got a hold of it.
Tobias and Tallie are right, and there are other things to look for:
As scum on top of the beer, or a ring around the neck is a nearly always from an infection (the other possible cause would be unusual ingredients, like cocoa butter from chocolate, but you would know that).
Another thing that can cause gushing, without infection or over carbonation is just having a lot of nucleation points in the beer. Suspended yeast will do it and (I think) chill haze and/or precipitated water salts can also act as nucleation points.
Gushing is a sign of over-priming. You can get over-priming from using too much priming sugar, bottling too early (before the yeast has finished) or from a wild yeast or bacteria that eats the remaining sugars and produces CO2.
In my experience, I have had a half a dozen gushers and two (unintended) sour batches. I would argue that all my gushers were from bottling before the yeast had finished, especially since one of my unanticipated sour beers was completely flat.
I live at altitude and suspect some yeasts just simply take longer here than anticipated; typically 3-4 weeks for S04 for example. It's a bit difficult to determine exact anticipated final gravity, since both wort ferment-ability and potential yeast attenuation will both limit final gravity.
I find using a temperature controlled fermentaion chamber, slowly increasing fermentation temperature over a two week period and adding a longer conditioning period has helped, and I'd argue improves the flavor as well.