...if someone really can doctor a file properly, how would you know the difference?
She missed a chip mat. That alone is enough to take a second look.
In general though, at a certain point, it is impossible. As detailed above, you can completely fabricate an entire ride by hand. Getting more advanced, but certainly easy enough for a clever coder, you could feed a path into some software which would the learn the elevation profile. You could tell it your desired average power or speed, usual cadence range and some sort of graph of your usual heart rate vs. power. All of these (location, power, speed, HR, cadence) could then be included in the final FIT file with some believable amount of variation.