I want to code the lost focus of a server control with clien side script (javascript).

In the textbox i write a employeeid, and I want that in the lostfocus call a server method to look for the employee name in the database, and show it in another textbox. I know that __dopostback, could help.

What I am trying to figure out is how can i Know in server code, which was the control that generated the postback?

Any ideas?