The halter, surcingle, etc systems depend on a level to determine when the horse is laying down. These can be easily fooled when your horse is sleeping.
Systems like Foal Alert work by sewing (usually by a vet) a magnet into one side of the mare's vulva, and a transmitter in the other side. When the magnet is separated, the alarm sounds. This means you very rarely have false alarms (usually when the mare itches her butt on a wall.) However, by the time the alarm sounds the foal has already peaked out of the mare's body. That means it may be too late to reposition the foal if there is a serious dystocia. Also, a red bag wouldn't give you much notice to run and save the foal.
Cameras are great but you have to stay up all night long and watch them for possibly weeks at a time.
Milk calcium kits test the amount of Ca in the milk. This level will spike a day or two before she foals. Some kits give you a little indicator to read (like a human preg test), which requires some interpretation.
What I recommend, from years of working with most of these systems, sometimes in combination, is a combo of cameras and a milk calcium tester called "FoalWatch" (Foalwatching.com). This kit, instead of the indicators, gives you a specific number (quantitative test). This number allows you to have a much better idea where the mare is and gives you an easy way to track changes (which are far more important than the absolute number). Using the Foal Watch system combined with cameras (in over 100 mares), I have never missed a foal being born, nor have I ever watched a mare more than two nights. HIGHLY HIGHLY recommended. I have seen sooooo many people here and in person tell me about how they watch for weeks and weeks and get no sleep, or how their alert system failed, etc. I hope we can all learn from their tragedies
