There is ABSOLUTLY no reason why you should be missing a Catalytic Converter. The only way it could be missing is if someone intentionally removed it, which is highly illegal and a Federal Offense. But to answer your question your check engine light would be on and DTC (Diagnostic Trouble Code) P420 Catalytic efficiency Code would be set. You will need a Scan Tool (Or a computer that can communicate with your car to retrieve DTC's).
If u are missing your converter move to Pahrump, Nevada. There is no emmission required here for auto registration and your only 67 miles west of Las Vegas! FUN capitol of the USA!!!!!! Just kidding, most small rural town due not require this test so maybe you live close enough! You need either a PO BOX or real address depending on where you live. My husband bought a 1 year old Ford mini cab long bed from Canada and they do not use CCE. Did you go to a CCE center? That's how we found out! Good Luck!