INSTRUCTOR AND STUDENT PLTS ABOARD C152 HAVE NMAC DURING PRACTICE DIVERSION.

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OUR ACFT WAS ON A REGULAR TRAINING FLT; THE STUDENT BEING ON ONE OF THE LAST FLTS BEFORE HIS PVT PLT CHK RIDE. WE WERE PERFORMING A DIVERSION FROM CUYAMACA LAKE TO ON THE ROCKS PVT ARPT; A COMMON DIVERSION FREQUENTLY CHOSEN BY LCL EXAMINERS FOR THE CHK RIDE. NOT MORE THAN 1 MIN AFTER OVERFLYING ON THE ROCKS; I NOTICED A HIGH WING RETRACTABLE GEAR ACFT EMERGE FROM BELOW US; HEADED TOWARDS OUR 1-2 O'CLOCK POS. HORIZ SEPARATION BTWN THE 2 ACFT MUST HAVE BEEN NON-EXISTENT; VERT SEPARATION COULDN'T BE MORE THAN 50 FT. SINCE AT THAT POINT THE 2 ACFT WERE ON DIVERGING COURSE; NO EVASIVE ACTION WAS TAKEN FROM OUR SIDE; WE REMAINED S OF SEE LOC-D CONTACTED SEE TWR APPROPRIATELY; AND CONDUCTED A SAFE LNDG AT SEE RWY 27L. ON TWR FREQ WE HEARD AN ACFT REPORTING AN NMAC; AND AFTER VERIFYING THE VISUAL FEATURES OF THAT ACFT MATCH WITH OUR OBSERVATION WE CONFIRMED OUR ACFT WAS THE OTHER PLANE INVOLVED IN THE INCIDENT. WX CONDITIONS AT THE TIME OF THE INCIDENT WERE VMC; NO CLOUDS; VISIBILITY P6SM. WE WERE COMMUNICATING ON FREQ 122.75 MHZ; THE CTAF FOR THE SAN DIEGO PRACTICE AREAS. NO EVASIVE ACTIONS HAD BEEN TAKEN FROM MY SIDE; SINCE BY THE TIME I HAD VISUAL CONTACT WITH THE OTHER ACFT IT WAS 'ALREADY TOO LATE.' CORRECTIVE ACTIONS: 1) IN THE FUTURE I WILL EXERCISE EXTRA CAUTION WHEN XING ANY IFR APCH PATH; AND IN THE IMMEDIATE VICINITY OF THESE APCHS. 2) I WILL TRY TO FLY WELL CLR (ABOVE OR BELOW -- AIRSPACE AND TERRAIN PERMITTING) OF THE APCH ALTS. 3) WHEN FLYING IN THESE AREAS I WILL CONSIDER MONITORING THE APPROPRIATE ATC FREQS (IN CASE DUAL RADIO UNITS ARE INSTALLED); WHILE ON AIR-TO-AIR.