Question

How do you identify α and β forms in sugars?

Answer 1

α and β forms of sugars are a pair of stereoisomers. The only difference lies in the 3-D arrangement of an atom or group. The α form contains the $OH$ group of anomeric carbon at downside position and β form contains the $OH$ group at upside position.

Complete step by step answer:
Before going into the forms of sugars first we have to be aware of the anomeric carbon of sugars or carbohydrates. The anomeric carbon is the characteristic carbon present in the cyclic forms of sugar molecules which is present in the carbonyl carbon in the open chain of the carbohydrate. The process of conversion of one anomer to another is known as anomerization.

The cyclic form of carbohydrates is known to exist in α and β forms which are so labeled by considering the position of the substituent hydroxyl or alkoxy group at the anomeric carbon. The $6$-membered ring is considered as a flat ring. Then the relative positions of the $C{H_2}OH$ group and the $OH$ (or $OR$) group at the anomeric carbon is to be determined.
If the $C{H_2}OH$ group and the hydroxyl group at the anomeric carbon are on opposite sides. This indicates that the $C{H_2}OH$ group is in the top position and the $OH$ group is at the bottom position, then this anomer is referred to as the α-form of sugar.
If the $C{H_2}OH$ group and the hydroxyl group at the anomeric carbon are on the same sides. This indicates that the $C{H_2}OH$ group is in the top position and the $OH$ group is also at the top position, then this anomer is referred to as the β-form of sugar.
Thus in the above figure the starred marked carbon containing hydroxy group is the anomeric carbon. The cyclic form in the left side is the α-form of glucose and the cyclic form in the right side is the β-form of glucose.

Note:
When disaccharides are formed from monosaccharides the exact position of the hydroxyl group at the anomeric carbon determines which disaccharide is formed.