Do we know what a health certificate means in this context? Could it be as simple as a temperature check?

The blood tests that can be done cheaply/quickly don't show if the person is currently infected, while the full blown test that requires a sample from the nose is impossible to get pretty much anywhere. Especially if you are without symptoms nobody would give you such test. Even if they were, it would be expensive.. Private places doing the test are charging around 200 euros per test.