For an iPhone user:
Launch Settings from your Home screen.
Tap on Battery.
Wait a moment for Battery Usage to populate.
Tap on the Show Detailed Usage button to get a breakdown of foreground and background power usage.
Tap on Last 7 Days to get a broader look at power consumption over time.
For an Android user:
In all versions of Android:
Hit Settings > Device > Battery or Settings > Power > Battery Use to see a list of all apps and how much battery power they're using.