The United States would detect a leak, at Chernobyl or anywhere else. We have monitoring aircraft that are used to sniff out covert nuclear tests, and their sensors are ridiculously effective; they would be able to tell what had happened, and distinguish between a bomb and a reactor accident. Whether they would publicize that information depends on the circumstances; for example, the US government knew about Kyshtym when it happened but didn't tell anyone to avoid hurting the emerging US nuclear power industry.
Russia and other groups have similar setups - including the IAEA, if I recall correctly - but I'm not sure when they were established. And there are various other pathways that could lead to discovery. These days, it's essentially impossible to hide an accident that results in a radioactive release.