A simple question: How to determine the rf frequency with a LO, in the IF band ?

I have thought such a simple application for several days. In a downconverter system, take the LO frequency is 3500MHz for example, and the IF is 600MHz, then if there is no prefliter in the rf path, how can I determine the rf is 2900MHz or 4100MHz? In the mathmatical process, such two rf signal have different phase by the LO frequency. Can I pick the phase information out so I can determine the rf frequency without a image-frequency filter?

Using a standard mixer the RF could be either of the situations.

If you use an Image-Reject Mixer you can make the difference:

http://www.triquint.com/prodserv/tec...SSB_mixers.pdf

In communications system design, RF frequency selection usually comes first. Certain RF bands can be used "free", others with a license only. The aim is to properly use the frequency spectrum without causing or be bothered by interference.
One cannot simply transmit any signal "in the air" as someone else can be using it, often the other party paid for the right of using it.

The good quality upconverters do have a good image rejection. You can use IR mixers which only output one selected sideband, or you can use filters to reject the unwanted spectrum. Mixers also generate harmonics, so low-pass filters are used to clean output spectrum.