After 21 years of silence how and by whom was OSCAR-7 first identified as functional?

Oscar-7's designed lifetime was three years.

But it did work from 1974 to 1981 (in its first period), about the double designed lifetime.

Oscar-7 was reliably used by the amateur radio community all over the world from just after launch until June 1981 when its batteries likely shorted. This in turn short-circuited the entire power system, just like shorting your car battery would short out your alternator and deprive your car of any power. This prevented any power (including that from the solar panels) from reaching its repeater circuitry. The satellite remained dormant for just over two decades until what seemed to be a miracle occurred on or before July 2002.

In late July 2002, an amateur radio enthusiast in the United Kingdom (callsign: G4CUO) heard familiar signals from what he believed to be the "silent" Oscar-7 satellite. A quick check confirmed that Oscar-7 had indeed been somehow revived from the dead and was transmitting once again. Soon after this sudden and amazing revelation, amateur radio enthusiasts were once again using Oscar-7 for communications after a 21-year hiatus. It was believed that the electrical short in the batteries had somehow opened up (due to thermal extremes?), once again permitting the solar panels to provide power to the onboard repeaters. At that time, the satellite was nearly 30 years old, which is very long in the tooth relative to most satellite missions and nearly 10 times longer than the designed lifetime of the satellite.

Source: http://castor2.ca/07_News/headline_072914.html

Im Sommer 2002 wurde ein britischer Funkamateur auf ungewöhnlich starke Morsesignale im Satellitenbereich bei 145 MHz aufmerksam. Eine Auswertung der Signale ergab, dass dies die Telemetriebake von AO-07 war. Sie übermittelte Messwerte wie Temperaturen und Ströme aus dem Satelliten.

Source: https://amsat-dl.org/amsat-oscar-7/

Translation: In summer 2002 a british radio amateur noticed extraordinary strong Morse code signals within the satellite signal range at 145MHz. An examination of the signals found this was the telemetry beacon of AO-07. It transmitted measurement values like temperatures and currents of the satellite.

So the radio amateur G4CUO rediscovered oscar 7 just by accident and by remembering to signals heard twenty years ago. I guess he heard the telemetry signals of oscar 7 sended using Morse code.

Experienced radio amateurs with a lot of Morse code practice are able to recognize the individual hand writing of other amateurs.

The telemetry looks like that:

hi hi 
100 145 176 156 297 245 200 254 370 34x 328 354 453 455 450 451 542 501 553 529 
60x 601 601 651
hi hi 
100 166 179 156 297 263 201 254 376 368 331 354 448 455 449 451 541 501 552 529 
60x 601 001 651

The telemetry does not contain the call sign W3OHI, so the call sign could not be used for identification.

Independent of G4CUO AO-7 was found by Pat Gowen, G3IOR on June 21, 2002 as Blue Coder noted.

But every detection required a receiver tuned to the satellite frequency range 145 MHz and a directional antenna with the proper azimuth and elevation to the satellite.