You are correct that most carriers use a different frequency for uplink and downlink; however, 3G and 3.5G operate on the same frequency (for incoming and outgoing data, respectively) since it is technically based on the same technology.

So to go back to the original question, you can measure 3G signal using app like Netmon but you cannot measure 3G and 3.5G separately since the tower will only connect you to one standard at a time.