I was discussing with a friend about reversing an fpga-board which comes with an unknown .ucf, and he claimed that the .ucf can be determined without desoldering anything.
His argumentation is based on
this article!
The tool mentioned in the article can detect connections between multiple JTAG-enabled devices on the same board. It basically uses standard boundary scan.
It will have very limited (or none at all) usability for a board with a single FPGA. It will also not detect connections for LEDs, switches, and other external components.