According to Pierre Sprey, the technology to detect stealth aircraft (e.g. the F-35 Joint Strike Fighter in air by radar has been around ever since the Battle of Britain (i.e. 1940, early World War II).
Source: Interview with Pierre Sprey:
Interviewer: How stealth capable is the F-35?
Pierre Sprey: The first thing to know about stealth is it is a scam. It simply doesn't work. Radars that were built in 1942 can detect every stealth airplane in the world today.
Is this true?