Was wondering about the topic, then realized technology would be progressing towards this if we don't have it already. Are there any consumer fitness trackers or similar devices currently on the market that could detect whether or not the wearer was experiencing microsleeps
This is probably possible to some extent, but I'm not aware of any products that do it. Signs like head bobbing and eye movements are easily detectable these days---hooray for cheap sensors.
On the other hand, ECG and fMRI are utterly impossible in portable tech. Those types of machines range in size from "kitchen appliance" to "walk-in closet".
This might not be possible for a watch or a smartphone, as the sensors would require constant detection of the head and face. Google Glass or a head-mounted AR kit could presumably do it.