Easiest thing to check is the cap on the rad, make sure the seals are in good shape, the cooling system needs to be under pressure. If the car has an electric fan, check that it is working as well. Mine was seized and would cause the car to heat up to the 3/4 mark if you sat in traffic for 10-15 mins. If these things check out, I'd say the water pump may be your culprit.