How do I decide the electromeric effect on pent-2-ene?
Here what you should consider first is hyperconjugation, rather than inductive effect (a brief explanation by me).

You're right that $\ce{Nu-}$ will either attack on $\ce{C-3}$ or $\ce{C-2}$, and hence two carbocations would be formed. Now, we'll analyse the stability of these carbocations on the basis of hyperconjugation which is as follows,


carbocation at $\ce{C-2}$ will have a total of 5 HC structures
carbocation at $\ce{C-3}$ will have a total of 4 HC structures


Based on the above difference, it is easy to conclude that $\ce{Nu-}$ will attack on $\ce{C-2}$ carbon. Whereas based on inductive effect, those carbocations are nearly equally stable.



Note: HC is short for "hyperconjugative structures".