How to confirm WiFi data is being used, not data plan? How can I confirm that I am using data only on WiFi?

In other words, I am connected to WiFi at home and work, yet I get notices that I am close to hitting my data plan's max limit.  Is there a way to somehow confirm that my data is being sent via WiFi when at home/work?

When connected to WiFi, even if the signal strength is say 1% of the WiFi connection, it's my understanding that my cell data plan will not be used until the phone disconnects from the WiFi, correct?

I know there's a way to see the amount of data sent over WiFi and the Data Plan, but I'm wondering if there's a way to somehow confirm that I'm on WiFi and the data is actually being sent via WiFi, and not using my data plan at all (without turning off the Data Plan in settings).

I'm just basically trying to confirm that my data is all going over WiFi, and not Data Plan, when connected to a WiFi Router (with any signal strength).
I use a Galaxy S5 running Android 6 (Marshmallow) and my phone has different icons for wifi and data.





My phone also has little arrows that turn white if there are packets are being uploaded or download.

You can see data usage by app in the data usage setting.



And if you find that an application decides to use a lot of background without telling/asking you, you can click on it, and disable background data.