# How can you determine carbocation stability?

Carbocation stability is mainly due to hyperconjugation of the carbocation by inductive sharing of electron density from adjacent alkyl groups filled with $C - H$ bonds,
The methyl carbocation doesn't have nearly any hyper conjugation so it's going to be very unstable, whereas the tertiary carbocation will be relatively stable due to the large amount of potential electron density to inductively participate in its $\text{p}$-orbital.
For the preceding reasoning, SN1 reactions will favor tertiary $\alpha$ carbons, whereas SN2 reactions will favor methyl $\alpha$ carbons.