They've been well known since that a F-117 was shot down in 1999 during the Serbia conflict. The actual event was mostly due to bad operational tactics on the American's side and a very lucky shot, but it did highlight that it was possible to detect stealth fighters using coldwar-era Soviet technology.
The problem is that the stealth is specifically geared towards XHF frequencies, which are the frequencies used in radar guided missiles.. which are the favored missiles for the USA.
However the planes lack the physical size and features necessary to completely absorb VHF radar. VHF lacks the resolution necessary for guided missiles and VHF radar stations must be very large, but it can be used to monitor air traffic over vast areas.
So if you detect a aircraft on VHF and then point your XHF radar at it and nothing shows up... Then you know you are dealing with a USA stealth fighter.
Stealth has been heavily oversold to the American public, but foreign governments are not so vulnerable to USA industrial-military propaganda.
This doesn't mean that stealth is worthless. It does dramatically reduce chances of detection, just doesn't make them invisible. They are still largely invulnerable to radar-guided missiles. The size of VHF radar arrays needed precludes their use on other aircaft.
So to make this sort of thing still requires a sophisticated network connecting large and easily found radar stations, which themselves can be targeted and disrupted in a combat environment. There is still major hurdles needed to solved before it would be possible to actually shoot down these aircraft with radar-guided missiles.
Also this technique doesn't work as well against aircraft the size of B-2 bomber.