The power within a programmable device is variable depending on use, so an upfront consideration should be made towards using the device for thermal testing and characterization. Of course it is best to use the final device programming image to do overall thermal characterization as it most accurately represents the power in the product. That is not always possible because often the board is designed and completed prior to the final image. The programmable nature of the FPGA allows you to create a power consuming design for the purpose of thermal testing if a more accurate image is not available. Such a design generally is not functional but dissipates power to whatever levels desired for testing. There are several methods to design such a test image. If such a test image is required, it is suggested that the image is requested from Xilinx. Depending on the target device, there can be specific considerations to understand.