I got this from a horse colour expert on another website
Sabino is thought to be polygenic - that means controlled by more than one gene. It can be expressed as minimally as one small ragged sock and a white chin, right through loud sabino (where markings are easily confused with some overo patterns) to maximum sabino, which appears white - occasionally with small spots.
There are several characteristics that sabinos show - in order to be classed as sabino, a horse must show at least two of the following;
On legs - ragged white socks or stockings, often ending in a point; knifeblade socks; partial socks or stockings; spots on the legs
On head/face - wide, irregular blaze; odd shaped white on face; lip spots; white chin; white spots under throat
On body - splashes of white on the belly, or extending up the flank or shoulder; odd white patches, especially 'lightning strike' markings; roaning, which can be a little on the belly or flank or cover the body and even head and legs.
Sabino is often better expressed on a red base - so often shows more on a chestnut (or even bay) coat than on black.
Tobiano is the gene responsible for what most of us know as coloured horses. It causes white patches on the base colour, and is what's called a 'simple dominant' - that means if a horse carries the tobiano gene, it will show tobiano colouring.
Tobianos usually have dark heads, white legs (up to the knee at least), white at the top of the dock and dark at the end, and can have ermine marks, spots, and roaning or bleeding of colour at the edges of patches. The spine is often white (although minimally marked tobianos sometimes have no more than a small patch of white on the neck or shoulder) and the flanks often dark.
Extreme white markings on the face are not down to tobiano, even when they appear on a tobiano horse - they are the result of another gene (or two!).