At first you wouldn't know it.. the pain wouldn't come until a little later - might be hours, might not be until the next day .. there would be pain, weakness, nausea, etc.. there are a number of sites that give details of an acetaminophen poisoning ... it's not fun!
But if you're asking I sense you might be afraid you took to much.. if this is the case, how much did you take? You probably have nothing to worry about...