how do we determine whether a reaction is stereospecific or stereoselective??

Stereoselectivity is the preferential formation in a chemical reaction of one stereoisomer

 over another. When the  product  stereoisomers are enantiomers, the phenomenon is

called enantioselectivity and is quantitatively expressed by the enantiomer excess; when

 they are diastereoisomers, it is called diastereoselectivity and is quantitatively expressed by

 the diastereomer excess

Whereas  stereospecific reactions are those in which diff . stereoisomers of reactant   leads

to diff . stereoisomer s of product .

stereospecificity describes a reaction that necessarily yields a given

 stereoisomer because of the mechanism of the reaction, while stereoselectivity

 describes a reaction where the mechanism allows for the formation of both

 products, but the formation of one of the products is favored. Stereospecificity

 always yields 100% of a particular stereoisomer, while stereoselectivity does

 not necessarily give 100% of the favored isomer