They will have to have one, as they will want to keep illegal goods out of their terrotory, just like all countries do.Then it will be up to the EU to erect a border if they want one.
How do they work if the border exists only as a line on a map?As for detecting foreigners, there are already border checks which work effectively.
Of course.Why would a Latvian want to enter the UK illegally anyway, they won't be able to get a N.I. number or access the benefits system without proving entitlement.
Now that you say that, that explains why there are currently no illegal immigrants here.