How do budget airlines check visas when you check-in online?

** Will my travel documents be checked at the airport?**

...

If you check-in online you will still need to provide your photographic ID when you go through security and again when you board the plane.

http://www.easyjet.com/en/help/preparing-to-fly/travel-documentation

It is at that point they check the visa requirements, see the section following the above for Easyjets comments on International Travel and travel documents.

How do budget airlines check visas when you check-in online?

They check your documents at some point before you board.

• If you check baggage, they will check you documents at that point.
• If you proceed directly into the terminal, they will check at the gate or at a special desk somewhere in the terminal.
• If they need to verify documents, passengers may be called to the gate stand.

Note, they may check more than once.

Regardless of how or where it happens, they will be checked. Check-in is a somewhat minor formality and is really unrelated to document verification.

With at least one budget airline, who shall remain nameless, in the old days when my wife (a US citizen) was living in the UK on ILR, she simply wasn't allowed to do online check-in. We were required to check-in in person, although the fee for not checking in online was waived.

Now she has a UK passport, and is travelling in Europe on that, things are much easier (at least until Brexit).