How to know which phone number Whatsapp is using on my phone? On my phone I used my T-mobile number as well as my Google voice number. I registered to Whatsapp using my Google voice number. How can I check in Whatsapp which number it uses?

I use a Samsung Galaxy S5 (SM-G900A) with Android 4.4.2.
The number WhatsApp uses is the one you registered with; to see it go to menu (the three dots at the top right corner) -> account -> payment info and there you will see which number is associated with the WhatsApp account.

EDIT: Since WhatsApp removed the payment, it is possible to check the connected number by opening the settings menu and tapping on the badge displaying name, status and profile picture.