Duct9143
Obviously not matchboxbananasynergy, but here's an answer: you can see what apps (including Signal) are running all the time by doing the following:
- Swipe down from the top of your screen twice (first you'll see notifications, then after the second swipe you'll see quick settings tiles)
- At the bottom of your screen, you'll find 3-4 circular buttons at the bottom. Tap the one with the number in it.
- A window called "Active apps" will pop up. All of the apps in that window are running in the background.
I can't say 100%, but if you see Signal there, it's almost certainly because it is handling notifications on its own, and not via Sandboxed Google Play.
You may also see a persistent notification from Signal saying something like "receiving messages" or something like that.