1. Bowie was left handed. When signing an LP or photo, a David Bowie signature (almost always) tilts downwards FROM LEFT TO RIGHT with the B on the left being higher. There are a few exceptions (ie when signing prints etc when it is often level). Rarely does Bowie sign upwards, as right-handers tend to do.
2. The date can also help determine the authenticity. Bowie's distinctive numbers are 4,5,7 and 8. His 9's always changed depending on urgency. Forgers have failed to work this out...look closely at the way he writes his numbers in the galleries as well as looking at the size of the numerals.
3. The bigger the space to sign, the larger the David Bowie autograph. Dependent on time, mood and surface (or lack of) to sign on , Bowie's autograph would tend to be big when given a big area to sign (LP/Poster) and sometimes crammed small on a photo. Sometimes there are slight variances when connecting the letters together, for instance when he doesn't join the B to the O as a continuation. This is rare but was prominent in 1983, 1995 and occasionally thereafter.
4. It is unusual to see an undated Bowie signature after 1975 unless he was in a pressurised huddle of autograph hunters or literally signing while walking. I estimate that less than 10% of David Bowie signed items where left undated, but of course, they are out there as can be seen in the galleries.
5. Bowie signed as "Bo" and year for fans, "David B" for friends/associates and employees and "David R Jones" for legal contracts/letters etc. It was VERY unusual to see him sign David Bo (or Bowie) post 1975 although there are some rare exceptions.
6. Bowie had favourite areas to sign on different LP sleeves - generally he signed in the same areas throughout his career. Check the galleries and BLOG to see this.
7. Huge variations could occur depending on the surface Bowie was signing on. You will see major differences in Bowie's signature for the same year - what you cannot be aware of is if he was signing on a table, signing to someone proffering an outstretched piece of paper or in the middle of a "scrum" being jostled by fans. As mentioned earlier, the numbers he writes for the year can help enormously to check authenticity.
8. Personalised messages and salutations. As mentioned, if he had the time and there was space to do so, Bowie would often write a message. His hand writing is very distinctive and there are quite a few examples in the galleries and BLOG. Note the way he writes "For" and "Best wishes" both rushed and unrushed. Typical salutations are "Best Wishes", "With thanks" and pre 1975, "love"