EuroTrash

I don't see why this is an issue. They won't know, but why do they need to know.
Under freedom of movement, EU citizens are only supposed to spend 90 days consecutive in another EU country as a visitor, but that has never been policed. For EU citizens and residents nothing will change, they will still be free to travel as they wish.
The 90/180 day rule is what's important. That's a Schengen thing not a country thing, and it is policed, by stamping passports. It doesn't matter which Schengen border you cross, if the stamp in your passport shows you entered Schengen more than 90 days ago, the exit border check will pick it up.

Moses2013

Kempf
Facts
Kempf is a German national working in the Netherlands as a music teacher. He works 12
hours a week. His income is not high enough to live on so he claims Dutch supplementary
benefit. When he applies for a Dutch residence permit, this was denied on the ground that
his income is not sufficient enough to support himself.Question
Does the fact that Kempf claimed financial assistance in order to supplement his income
exclude him from the provisions of art. 45 TFEU?
Judgment
A person in effective and genuine part-time employment cannot be excluded from art. 45
TFEU ​​merely because he seeks supplementation for his remuneration which is below the
minimum means of subsistence.
So: One the effective and genuine nature of the work is established it is irrelevant whether
supplementary money is obtained from financial assistance from public funds
https://www.studeersnel.nl/nl/document/hogeschool-inholland/international-law/samenvattingen/case-law-workers-lawrie-blum-bosman-lyon-levin/5453922/view

missile

Your passport will be checked on entering and leaving EU by air or sea.

Pulaski

Right, but without connected passport computers at the Schengen perimeter the French don't know whether you had spent more than 90 days in the Schengen zone recently, as you could have left by ferry from Spain, Belgium, the Netherlands, or Scandanavia, and so post end of the Brexit transition (the French) would not be aware of any reason to not let you into France even though you were living as an unregistered resident in Spain.

missile

I was responding to this statement
"How would anyone know if someone is legal or not between eu countries? or for that, even entering the UK or Morocco. We drive, so we don't fly or use the trains maybe that is significant"

Pulaski

OK, right.

EuroTrash

It's the same for self-employment. The EU criterion is that it's a "genuine and effective activity, and not on such a reduced scale as to be merely ancillary" or some such wording. As long as your business is considered genuine and effective, under EU rules you qualify for the same income support and benefits as any other resident of that country
If your business is classed by the authorities as a hobby business, that doesn't apply
Self-supporting inactives are supposed to be self-supporting and as such, not need support from public funds.
Freedom of movement was originally all about workers. It was extended to nactives and retirees later on, as a benefit of being an EU citizen, but the basic purpose of the scheme is to have a mobile workforce that can cross borders to go to where there is work available.

Moses2013

If you look closer there are actually many cases.
Udo Steymann was a German plumber working in the Netherlands. He joined the Bhagwan Community, a religious group who provided for each other's material needs through commercial activity including running a discothèque, a bar and a launderette. He participated in the community by doing plumbing, household duties and other activities. The community would provide for people irrespective of the activities they undertook. He applied for residence to pursue the activity but was refused. When he appealed a reference was made to the European Court of Justice (ECJ). The ECJ held that remuneration may be indirect “quid pro quo” rather than strict consideration for work i.e. work does not need to be paid for in money as long as the worker agrees to receive something else in return.

EuroTrash

The principle of workers having rights is qrystal clear. The issue that arises is, what constitutes a worker. States tend to want to be restrictive in their interpretation, and will often claim that a person whose work and remuneration is minmal, is not a worker because they say their work is not "genuine and effective". As you say there are loads of cases where the EU has reversed that decision and said the person is a worker and therefore has rights under FoM rules and their appeal has been successful. But there are also loads of cases where the EU has turned down the appeal and agreed that the person is not a worker, they are in fact an inactive person with an ancillary activity, and as such, if they don't have sufficient income to be self supporting they have no rights under FoM rules.
The issue of whether a person should be classed as a worker or not is probably one of the commonest disputes where FoM is concerned. But frankly, I don't think the average Brit early retiree who came to Spain for an easy life and wanted nothing to do with rules and bureaucracy and suchlike, would want to go through the performance of being refused residency and getting a lawyer to take their case to appeal in the ECJ. Apart from anything else, if they've never been registered to work and never been employed, there is no case to be made.