How To Check If Someone Is Secretly Reading Our WhatsApp Conversations?
Open a WhatsApp Web session on your tablet or phone by tapping the vertical dots icon in the main WhatsApp window.
Click on the WhatsApp Web option and you can check for any device or App which has access to your WhatsApp.
What to do if WhatsApp is Open on Another Device?
WhatsApp → Menu options at Top Right → Settings → Account → Enable 2 Step-verification.