Testing on unprogrammed blank FPGAs?

Yes, but there are some methods to GUESS if your fabricated device has something inside that is not yours. There are also some methods to prevent another team (the fabrication foundry team for instance) from changing or adding something new to your design. This is a field of research. Different approaches may use delay, temperature, power, parasitics, etc to detect a trojan. Nonetheless these methods can be applied if YOU have designed (even with third party tools) your chip.