How did Saruman know who Gimli was ?

[Inziladun]

Also, it appears Saruman had been spying on Aragorn, Legolas, and Gimli previously.

Suddenly Gimli looked up, and there just on the edge of the firelight stood an old bent man, leaning on a staff, and wrapped in a great cloak; his wide-brimmed hat was pulled down over his eyes.

TTT The Riders of Rohan

Gandalf, of course, later denied the old man they had seen was he, saying they must have seen Saruman.