One way to be sure is when you receive an email from the JAMB portal to confirm your registration. Apart from the fact that emails could be deceptive, or else you are sure of the sender, sometimes emails fail to be delivered.
The best approach to ascertain a successful registration is to ensure that your details have been entered directly on the JAMB portal and they actually appear on the portal. Once you are able to see your actual registration data on the JAMB portal, then you can relax.
Note that the importance of this procedure is to ensure you have been registered, before the actual closing date of 5th May. If you can’t confirm your registration, there is still time to make amends before the closing date. This procedure does not concern re-printing of exam slip for venue and date because that will most likely come after registrations.