first check solvent ...if it is polar protic then it is unimolecular.[E1/Sn1]...and if the solvent is aprotic..then its bimolecular[E2/Sn2]
first u have to check whether the atacking species in a good base or not...
then after check for high temperature....
then if stable carbocation is formed..which is in the order 3*>2*>1*...then its E1 and think of posible rearrangement too..