How do you know if you have low iron and b12?

Blood test. There are some symptoms that can clue you in about a possible problem but the confirmatory test is a blood test.
Blood test. A simple blood test ordered by your doctor will check for your iron and B12 levels. Depending on the result and the interpretation (which takes into account your medical history), appropriate treatment with supplementation may be ordered. Other conditions can cause these to be low so your doctor needs to look at your whole picture.