At least in the UK it's a legal requirement to use marked glassware when serving beer, cider and wine. (Or to use a marked measuring cylinder and pour that into the drinkware, which is sometimes done for wine.)

It's also the law in at least some EU countries, although I haven't checked beyond Wikipedia: https://en.wikipedia.org/wiki/Fill_line

Measuring beer is serious business in Germany to the point the EU commission had to comment on a rule change that supposedly forbade the usage of steins for foaming drinks (it didn't, but Germany wanted to be extra strict): https://www.bayern3.de/bier-steinkrug-eu-richtlinie

https://germany.representation.ec.europa.eu/news/klarstellun...

https://en.wikipedia.org/wiki/Fill_line

Selling drinks in mislabeled containers should warrant a fraud report to your local consumer protection agency. A crowdsourcing app seems like the wrong tool here.

and crucially, foam has to be above the line.

There is some back and forth around whether it's legal to serve beer in traditional ceramic steins, where customers can not verify that the foam really starts above the line.

As I understand, it is legal in Germany, but only if there is visible signage that informs customers about their right to pour their beer into a marked standard glass to check the amount. Source (German): https://www.abendblatt.de/incoming/article402102835/wer-hat-...

In 1899, an association was formed in Munich to combat fraudulent pouring. It was banned by the Nazis and re-formed in 1970. They went around and measured beers. This post is its spiritual successor. German: https://de.wikipedia.org/wiki/Verein_gegen_betr%C3%BCgerisch...