Determining the mechanism:
It is usually fair to say that the nucleophilic substitution of primary alkyl halides will occur via the SN2 mechanism, while nucleophilic substitution of tertiary alkyl halides will occur via the SNl mechanism. Generally, secondary alkyl halides are much more likely to react via the SN2 mechanism, but it is not probable to predict this with certainty. The one way to find out for certain is to try out the reaction and see whether the reaction rate depends upon the concentration of both reactants (SN2) or whether it depends upon the concentration of only the alkyl halide (SNl).
If the alkyl halide is chiral, the optical rotation of the product could be calculated to see whether it is a pure enantiomer or not pure. If it is, the mechanism is SN2 where as if not, it is SN1.