This question is kind of for the chemists here, but I'm open to any ideas.
In this question Denny Conn, who is far more experienced than me, asserted that a beer is likely finished fermenting, despite being at 1.036 (from a start of 1.056). This would be a beer that's about 2.6% alcohol and 9% unfermentables. To me, that would be undrinkable. Many people suggested rousing the yeast and/or re-pitching to finish out the fermentation.
The important point is, how do you know whether that supposedly stuck beer has any fermentable sugar left? The yeast might have given up early, which is a common enough problem. Or maybe the mash temp was missed by a lot? Or maybe something toxic got into the beer, causing the yeast to die early.
Other than re-pitching and hoping for the best, is there any way to measure the simple sugars? I usually suggest tasting the beer, but the amount of hops will affect the interpretation (as will the taster). I'm hoping for a test as quick and clear as a starch test.