2-methylstyrene in dilute H2SO4, how do I know which double bond to open?

There is only one double bond in styrene that is reactive. The double bonds in the benzene ring are not reactive as they are not really double bonds but part of an aromatic system and as such the electrons are delocalized around the ring.

The addition of dilute H2SO4 to styrene follows Markownikoff's rule which states that: In an addition reaction of a protic acid HX ) to an alkene or alkyne, the hydrogen atom of HX becomes bonded to the carbon atom that had the greatest number of hydrogen atoms in the starting alkene or alkyne. The formula of styrene is C6H5CH=CH2. So the hydrogen will add to the CH2 group at the end giving a carbocation intermediate C6H5CH(+)CH3. The anion portion of H2SO4 will then be added to the carbocation center and eventually be hydrolysed to an OH group. The resultiing compound is thus the alcohol C6H5CH(OH)CH3.