plawolf
Lieutenant General
There are too many variables depending on the radar, weather conditions, the aircraft's angle of approach, etc. There is no way of knowing.
There is zero possibility of detection if J20s were never closer than a few hundred KM from the boarder with India, and there would have been no reason for the J20 to go that close and every reason for them not to.
China has the Su35, which has a more advanced and capable radar than the MKI’s BARS. That means the PLAAF should have a very high degree of accuracy projection for the maximum range at which the MKI could pick up the J20 (not greater than the range the Su35 could).
With the RCS of the MKI, there is zero chance the PLAAF would allow them to get close enough to a J20 to have any chance of detecting them, since the J20 and/or other Chinese assets would be able to detect them far before they have any chance to pick up a J20.
What more, all production J20s in PLAAF service have lumberg lenses attached during all known documented manoeuvres. Since the PLAAF would have already tested their RCS extensively, meaning there is no need for them to fly clean to give anyone even a remote chance to get actual combat config RCS profiles. So even if the PLAAF allowed the MKI to get that close to the J20 for some unfathumable reason, what they would have picked up would have been the lumberg lense, and not the J20.