1997 Kia Sportage Possible problem with the Rotors?

I recently purchased my vehicle the previous owner did advise me that it shakes when reaching about 40 MPH. She mentioned it was because the front rotor needed to be replaced. I'm wondering if the rotor is more than likely the issue and if it is should I replace it or see if I need to have them trimmed. What would be any other likely causes? Practically everything under the hood has been replaced and I have the receipts of everything to prove it.