I never had any specific symptoms, it was picked up on a routine blood test. They can normally determine from tests whether it’s likely to be caused by lack of iron or lack of B12 and from this decide if further tests might be necessary.
Had some tests to check wasn’t losing blood in intestines, then ‘treated’ with ferrous sulphate, I say ‘treated’ because 6 months of tablets 3 times a day didn’t make much difference. I was then tested for coeliac which was negative. I’m meant to still take the tablets but I don’t because the only difference I feel on them is they make me feel sick, feel fine without.