Depends on what you mean precisely. In the case of an education skill or saving throw roll, you pretty much have to tell them - that's at -2 because of such and such. If they are making a roll against a secret TN - well you don't have to say anything.
The only other thing you can do, if secrecy is important to you, is keep a record of the characters stats, and don't tell them what they are rolling against. More work for you though.