power consumption of a FPGA
Generally speaking most CPLDs and FPGAs will have multiple power rails that are used for different things, for example, there is usually a core voltage (Vcc), reference voltages (Vref) and bank voltages (Vccio).  To analyse these voltages, a tool is usually provided with the manufacturers design software, for Altera and Quartus II this is called power play, for Xilinx and ISE, the tool is called XPE (Xilinx Power Estimator).  These tools work by looking at your design, taking in a few details from you and calculating things like power consumption, switching noise and even decoupling requirements.

If you just want to look at the waveforms on each power rail, all of them will have decoupling of some description so your best bet is to identify a capacitor used to decouple the power rail you want and attach your probe across it.

Are you looking at a specific VERTEX-5 dev kit?

Hope this helps,

Gipsy