Good questions! Well phones are improving with new capabilities every day, sometimes with just software upgrades from Google/Apple, so it's hard to say how a normal person would really stay up to date with that! In terms of knowing what sensors are inside your phone, you can usually google your device model number and get the Tech Specs page. This will usually list the general capabilities of the phone including the hardware inside and the software it comes with.

what impact does the app have on battery life?

Very little impact. I cannot notice any battery life degradation for example. One reason for this is that it only checks the sensors every 15 minutes or so, and it does it in a battery-safe way. In fact, Google tracks this kind of behavior automatically and gives negative points to apps that consume too much battery.

So I have a strong incentive to reduce the battery impact of the app, and have done so!