There are quite a few ways to know if you have enough juice for Crossfire/SLI.
Manufacturers suggested PSU requirements:
The manufacturers have to cover their legal buttocks, so taking into account the many poor quality PSUs that are out there, they give inflated PSU requirements that are often far higher than what is needed from a high quality PSU.
Many websites include power consumption in their review process. Find reviews of your cards on similar platforms, and compare the power usage. Understand the power usage of the rest of your system, and your card specifically, and compare that knowledge to the rating and capabilities of your PSU. There are also many PSU reviews out there that can help you in understanding the true quality of your powersupply.
Buy a power monitoring device:
And do your own testing. You still have to have an understanding of what your PSU is capable of, but having hard power usage numbers at hand are invaluable.
Trial and Error:
Not recommended in general, especially with a poor PSU (Yours is decent). Modern HQ Power supplies have overpower protection and other features that protect the rest of your system in the event of PSU failure. This means that you are taking the risk of being in that small % of people that still experience systematic damage when their HQ PSUs fail. That being said, Plugging in a second card and just going for it will often answer any questions you have been unable to satisfy prior to installation. Although, with your components, there should be no shortage of information out there.
I don't know the intimate details of your PSU, but OCZ Modstreams are not crap, and my general understanding is that a quality 650w PSU is enough for powering a stock speed Phenom II quad + crossfire 4870s. This is information that has been gleened from the above sources over time.