She knew that the Tenth Doctor was about to regenerated because she saw him in "The End of Time", part two". Also, the fact that he was able to name Rani, Sarah, Jo and Clyde.
Plus, in all honesty, who else would have an entrance like that? It doesn't take an Investigative Reporter to figure that one out.
Indeed. Of course, not everyone recognises the Doctor. Sarah was really clever to've recognised him before Rani did.. Even without the sonic-screwdriver and the TARDIS. He could've been anyone. And yet, she doesn't demand who he is. She recognises him. Clever Sarah. Big, gold star with an A plus for being smart.
Also, Sarah Jane had personally witnessed a regeneration (Third Doctor to Fourth); the others who were present had only ever heard about it at second hand. Indeed, those others had (until then) each met only one incarnation of the Doctor -- Jo had met only the Third; Rani and Clyde had met only the Tenth. Having direct personal experience of regeneration would have meant Sarah Jane was far less disconcerted by the change in the Doctor's appearance, etc.