Assuming you're talking about something like this, you could stick it in beer, but it probably won't give you any truly useful information.
The sensors use an enzymatic reaction to determine glucose levels in the liquid. The enzyme, however, is specific to glucose and won't detect any other sugars (for this reason it's really not comparable to a hydrometer). Glucose makes up ~10-15% of the fermentable sugar in any wort (the rest being virtually all maltose and maltotriose) and is the first to be assimilated by yeast (often in a matter of hours).
The glucose sensor could, therefore, serve as a decent indicator of the very early stages of fermentation, allowing monitoring of the drop in glucose levels, and this may provide valuable information for those willing to try. But it's probably only worthwhile if you already have one around as I think it might be hard to justify choosing it over other indicators of early fermentation action (especially pH).
Basically, you'll be limited to only the detection of glucose (whether you measure wort, beer, sugar syrups, extracts etc), and in the broad picture this is not terribly meaningful information, especially for the average homebrewer.