If my spleen didn't work, would I be able to tell?

Probably not, maybe. If the spleen is not working, eg from injury from sickle cell anemia, it is more likely to become seriously ill from bacterial meningitis or sepsis (blood infection) from strep pneumonia, neisseria meningitidis, hemophilus influenzae (vaccines are available). You may have anemia or easy bleeding if abnormal spleen is holding on to red cells or platelets. You may have abnormal blood on microscope.
Hard. Just by yourself no , why wouldnot your spleen work kind of unusual question. After auto accident if spleen trauma its removed and pt does ok. In itp spleen is removed.