How can a synthesis reaction be recognized?
At school: from the reaction equation: if you have more than a substance on the left side and only one substance to the right, that is the representation of a synthesis reaction.
In the real world: if you mix two or more substances and you make these bodies chemically reacting until the transformation will end, and if you separate all the substances that remained in the vessel and if you will find one, and only one new substance, and some remaining excess of any of the reactants, well, you have made a synthesis reaction.
Example: you burn hydrogen with oxygen and you get only water.
Organic and industrial chemists consider "synthesis" every reaction in which you obtain a main or bigger substance (as a drug, or a poison etc.) and more secondary products from other raw substances.
For example: if you prepare sodium chloride by reacting hydrochloric acid with sodium hydroxide, HCl + NaOH --> NaCl +