It often helps to take the picture at a slight angle. A base can be polished flat, cut and polished flat, either with or without arrises (tiny bevels on the edges of a flat polished surface); flattened on a marver and the pontil mark polished out, or roughly ground out, if it has been moulded, it will simply be mould shaped - and if there are seams showing from the moulds, they can either have been left - or polished off. The shape of the base of the mould can help a lot in identifying a piece too.
Bottoms may look boring, but they contains a wealth of information, so even if it simply looks flat, we still want to see the flatness - and any lumps, bumps, scratches or nicks on it.