Joelson used a process of elimination.
He treated everyone entering the embassy as a suspect and eliminated them one by one. If they truly could be cleared of suspicion, he'd abandon surveillance. But as long as he couldn't completely rule out doubt, he'd monitor them closely.
Especially those whose suspicions grew the more he investigated—Joelson was just short of taking matters into his own hands.
Alas, the CIA was currently divided into two factions internally bickering, not yet at the point of civil war. Joelson had been granted more authority than before, but he still didn't have absolute control.
Moreover, the embassy didn't only house a CIA intelligence unit—there were offices for the Department of Homeland Security, the NSA, and even Grasse had his own office within the embassy.
The largest embassy in the Middle East was also the largest intelligence center, and that was no joke.