2 doctors weighed in: How early after I am exposed to mono can it be detected by the standatd test? 2 doctors weighed in Want a second opinion? Monospot testing can usually detect antibodies 2 to 9 weeks after a person is infected. It typically is not used to diagnose mono that started more than 6 months earlier. Testing is not important if you come down with symptoms after being exposed, because there is very little you can do in the way of specific treatment.