To validate the COD orders you can verify the mobile number of the user by enabling COD verification on mobile feature.
To enable go to
Apps section under advance settings.
Search "COD verification on mobile" and install the app.
Once the app is installed go to
Payment section under settings.

Edit the COD/ Cash on delivery payment method.
In the edit window, enable the validate option under settings and mark validation source as SMS.

Once the changes are made save the settings. After these settings are validated before placing the order user will receive OTP via SMS and he can confirm the order by entering the OTP.