Merchants usually display the "Verified by Visa" or "Mastercard ID Check" logos on their website.
It's possible that a merchant is enrolled and doesn't display the logo (or is more difficult to spot), but if they are enrolled in 3D Secure, when you make a payment you will be automatically directed from the merchant's website to a secure page where you follow the steps below:

Enter an OTP code received via SMS.
➤ If you have entered the SMS code correctly, continue to step 2, where you need to enter a static password (this is set the first time you make an online payment with that card and is valid for 90 days).
➤ After entering the OTP code and static password, you need to return to the merchant's page to finalize the payment.