Measure the rate of the reaction with benzene. Then see what happens to the rate when one of the hydrogens on benzene is replaced by a different substituent like Cl, CH3, NO2, etc.

If the electrophile does *not* hit the double bond, you are correct that a normal alkene should be slightly activating since it can act as a pi donor.

HOWEVER, One problem is that alkenes will react more easily with electrophiles than will the aromatic ring, so they are often destroyed during attempted electrophilic aromatic substitution. Attempted chlorination of styrene, for example, will end up hitting the alkene.

Electrophilic aromatic substitution can work when the alkene is in conjugation with an electron withdrawing group such as an ester, which helps to make the alkene more electron-poor.

Is there a specific example you are thinking about?

I will take that as a compliment, Steven. Thanks