In the scene in the novel just prior to the Sollozzo meeting, when Hagen is briefing Don C., he says the Tattaglia family "May have a piece of the action". (italics mine)

When he gives Don C. his reasons why he should accept the upcoming Sollozzo proposition, he is looking to the future and the enormous profits to be derived from the drug trade, which, if realized by one of the other families, could make them more powerful than the Corleones, and he tells Vito

"If we don't get into it, someone else will, maybe the Tattaglia family" (italics mine)

Then, at the meeting itself, when Don C. asks Sollozzo "What is the percentage for the Tattaglia family?", there is no "My compliments" comment from Sollozzo to Hagen.

So, it would appear that while the Corleones were not 100% certain that the Tattaglias were involved, Don C. took a shot that they were with his question to Sollozzo, and the question hit home.

Sollozzo probably figured that his deal with Tattaglia (and Barzini) was secret, and he became nervous that Don C. was one step ahead of him.