don't bother with closing background apps, 9.9 out of 10 it's not running nor is it consuming any memory
iirc there's only 2 service that allows an iOS background app to continue running, the location and audio service.
active location service apps is very easy to detect via settings.
audio, you can obviously hear it, unless the app developer decides to play punk and play infinite loop sound at zero volume when in background to purposely drain your battery