Two markers of a good personal trainer
1. Does he or she provide a model of the health, fitness, vigour and appearance that you want for yourself? In other words, do they display evidence of not only knowing the theory, but of having been through the regimen they will set for you, understanding both the mental and physical stress you will need to go though to achieve your aims?
2. In the Dumbbell Press, how does the trainer guide you? If he talks about correct form and guides your lifts with gentle pressure under your elbows or upper arms ... he's a real worry! The danger in this exercise is in the weights being too heavy to lift, the arms weakening and weights crushing on to your chest. Guiding the upper arm or the elbow will just send the weights inwards and downwards if you weaken. A trainer who knows his stuff will hold your wrists, without a tight grip, but braced for faltering and ready to assist the final one or two struggling lifts. This may not look as pretty and will require the trainer to be more active and alert, but you can be confident he knows his stuff.