It was pretty much an ongoing theme throughout both Buffy's show and his own that Angel tends to keep tabs on Buffy and the goings on around her. He doesn't keep a constant watch, because some things do slip by him (her affair with Spike in Season 6, for example) but she pretty much remains the object of his concern.
As for Joyce's death specifically, one of the Scoobies might have called him, or it might have been something as simple as him having a subscription to the local paper. My guess would be both in this particular case.