OK...so Bowie's 2001 autograph was pretty consistent in that year. I have put one of the fakes above alongside a genuine from from 2001.
Areas that give the fake away are ringed in red below:
1. The tight small inward curve at the start of the signature indicating its been done by a right hander.
2. A sharp cut back rise at the base of the B instead of the more rounded base as per the genuine example.
3. Large gap between the "o"and the finishing line and an untypical "2"
In addition, right handers tend to sign upwards (B low, the end of the autograph higher). Bowie (the majority of the time) signs tilted downwards (B high, end lower) - in the genuine example, for once, he has signed straight. Hope this helps.