I don't think its been directly explained, however two possibilities are
1) Widmore's daughter Penny had access to a remote station looking for a signal from the island (It came when Desmond turned the key). Perhaps Widmore had a similar team that noted something when 815 disappeared?
2) However more likely (IMO) is that Widmore had a relationship with Eloise Hawking (Daniel's mother.) After she killed the adult Daniel in the past (while paradoxically pregnant with him) she recovers his note book which detailed much of what happened in his adult life (including presumably some details about going to the island). Eloise (& Widmore) would therefor have know either exactly what plane would crash when (or at least roughly when to look for a plane that disappears - doesn't happen very often)
Thinking about it, it would appear obvious that Widmore didn't know exactly when/what plane was going to be involved as otherwise he (or some employees) would have been on the plane.