Well, details are classified, so specifics are not available. But it's pretty safe to say it's RCS is quite a bit lower than a B-1 or F-117, and probably lower than a B-2, if for no other reason than because it's smaller to start with. (and newer)
Stealth is a relative thing, it's more than just low radar observability. There's IR, EM, Visual, and Aural footprints to consider also.
In general terms, the Raptor probably has a radar cross section of a medium size bird. The JSF (likely) has a frontal RCS about the same as the Raptor, but it is more visible from the rear, due to the exposed engine nozzle. Obviously the internal weapons carriage helps reduce RCS on all of these AC. The exterior shape is designed to reflect away or scatter the radar energy that hits the AC, rather than reflect it back to the source. Exterior coatings and internal materials and structure are designed to absorb what is not reflectd away.
As far as IR, the Raptor has the others (except maybe the B-2) beat. Supercruising without afterburner and cooling the exhaust gases keep the IR footprint down until the burner is lit. A lot of effort has gone into minimizing the heat buildup on the skin of the AC through shape and heat dissipating materials, etc. I have heard that the heat on the leading edges is even sinked away and used to preheat the fuel.
The radar is a big factor, since it is the #1 EM emitter on the AC. Most radars emit high energy pulses in a narrow frequency band. The AN/APG-77 emits low energy pulses over a wide frequency band. When multiple echoes are returned, the radar combines the signals to get a target. Since the pulses are a much lower energy signal, it's more difficult to detect. And Raptors share data, so they can acquire and lock on targets without even transmitting anything. They can get targeting data from another Raptor or an AWACS operating away from the hostile airspace.
Electromagnetically, they can minimize their footprint by using tight beam transmissions between aircraft or sattellites, so they are not broadcasting any omnidirectional radio signals for passive detectors to pick up. Undoubtedly there are other tricks used to minimize the EM footprint also.
They can still be picked up visually (weather permitting), but they have a paint scheme that is designed to blend into the background and disguise the outline of the AC. The US is developing active coatings that can change hues based on the surrounding light hitting the AC. Whether these ever end up on the Raptor is anyone's guess.
There is much more that is not known, such as active radar cancellation technologies, EM masking, etc. It's anyones guess just how much tech is on the thing, but something is running up the price, and it's more than a neat shape and RAM coatings.
