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I've always wondered about this...
I can look up the exact chemical structure of (say) glucose, which is a sugar. That's easy enough. But what I cannot figure out is this: if I proffer some random molecule at you, how do you determine whether it can be considered a "sugar" or not? I can't seem to locate anywhere a definition of what it means to be a sugar; I can only find lists of specific molecules which are sugars.
There is no standard "rule" for identifying sugars because in chemistry there are always exceptions to any rule. Nonetheless, the following guidelines will help you correctly identify a simple sugar (a monosaccharide) in most cases.
Simple sugars have the molecular formula $\ce{C_{n}(H_2O)_{n}}$, where n is at least 3. They also have to be capable of forming an aldehyde or ketone carbonyl group. For example, in glucose the cyclic forms predominate, but there is an isomer with a carbonyl group.
Whereas compounds in the inositol family fit the molecular formula requirement, but are not considered to be sugars because they are incapable of forming a carbonyl.
Sugars are a family of compound that includes monosaccharides and (poly-/oligo)-saccharides. The latter are derivide from the former by means of inter-esterification. Monosaccharides include mostly linear-chain molecules with three to seven carbon atoms that consist mostly of $\ce{-CH(OH){-}}$ and one $\ce{-C(O){-}}$ units terminated with hydrogen on chain ends. Some C8- and C9-molecules are also considered to be monosaccharides
Monosaccharides often exhibit linear-ring tautomery, and polysaccharides are usually built from ring forms of monosaccharides.
Traditionally, some molecules including $\ce{-CH2{-}}$ block, like deoxyribose, are also considered monosaccharides, though it may be debatable.
While brutto-formula of sugars usually is close to $\ce{C_{n}(H2O)_{m}}$, it is not s sufficient criterion to find one. For example, dimethylcarbonate has formuls $\ce{CH3-OC(O)O-CH3}$, i.e. $\ce{C3(H2O)3}$, but is not a sugar.
The relevant wiki article http://en.wikipedia.org/wiki/Sugar
some common monosaccharides
