Quote from previous post:
"In lab, we weren't able to change the concentrations, so how would you be able to tell whether 2-chlorobutane and 2-bromobutane is doing a SN1 reaction or a SN2 reaction? "
Well, if you had optically active materials, the product of SN2 would still be optically active, whereas SN1 would be racemic But most undergraduate labs do not use optically active materials (at least not 2-halobutanes), so this will probably not be of use to you.
In the absence of experimental data, the general rule is that (for secondary halides) strong nucleophiles in aprotic polar solvents favor SN2, whereas weak nucleophiles in protic solvents favor SN1 reactions. But as previous contributors to this post have said, their data do not seem to support this trend. Contamination (esp by water) can cloud the results.