First of all inspect the equipment in the gym. Any gym has at least one day free pass, so you can get to any gym for free and look around. You start with the equipment. Make sure that the gym has enough machines which are not packed too close to each other, that there are plenty of dumbbells of different weight. Make sure that the equipment is safe to work with and doesn't have any rust on it and not damaged in any way.
Safety comes first then everything else.
When you come to the gym, you have a certain goal you want to achieve, whether it's gaining muscular mass or loosing weight or getting in shape for the summer or just trying to make yourself look and feel better. When you have a goal, it's easier for you to decide what machines you will need in purpose to achieve that goal. If you don't know which machines are needed for this or that muscle, ask a personal trainer to show and to explain which machine does what. Ok, now you know that you have enough or not enough weights to work with. Next thing you look at is how comfortable do you feel in this gym.