Any benefit in replacing O2 sensors?

[lsupcar]

My '92 with 140K miles has the original o2 sensors. No check engine light and several years ago my mechanic said they were working fine. Even before then, the mileage seemed worse then it used to be, 16 mpg city, and the exhaust seems rich in gas. If they are bad, will there always be a concrete sign such as a warning light? If I were to change them in the above conditions, should I expect to have any noticable benefit? Thanks. Bill Z