Only sure way to tell is the serial number. But seriously ... what bike would someone buy that Looked like the bike in question, which, with the application of Cannondale stickers, would suddenly be worth all the money? And whether the components were replaced or not ... you would be able to see what the components were, right .... by looking at them? And then you could judge the value.
Here is a point: If you really the think the seller is so dishonest that he would stick "Cannondale" stickers all over a cheap frame and try to fool someone ... why would you buy it?
If nothing else, I would worry about the wear and tear. It's not like anyone who would lie like that wouldn't lie about the age of the components. Seals, pivot bearings ... that stuff adds up.
If you really think the seller is that dishonest ... why not buy something else?