1. Dirty Gyms. Gyms are pretty dirty places and some are worse than others. Everyone is sweating, few people actually wash their hands and almost no one wipes off the machines. If your gym seems like it's the dirtiest one on the block, it probably means that it sucks.
2. Too Busy. If your gym is always busy during the time you want to go, your gym sucks. The only way to really prevent this is when you're thinking of joining a gym, visit during the times you're likely to use it. That way you can see for yourself how many people are actually there. In most cases, the problem lies with cardio equipment since you can't share it. You can always work in with people if weight lifting is your main activity.
3. Rude Staff. When you first joined, the staff was probably really nice. They showed youaround, answered any questions you had and went out of their way to help you join. Now that you've signed a contract you're stuck and they don't seem quite as helpful as they once did. If the staff at your gym is only motivated by sales and not by pleasing the customer than your gym sucks.
4. Bad Equipment. What's the difference between working out at home and going to the gym? The answer is equipment. If you notice that your favorite machine is always out of order and it takes forever to fix, then what's the point of paying for a membership? Your gym sucks.
5. Paying too Much. If you're paying too much, you need to start looking at other places. With gyms popping up all over the place, competition is starting to lower prices. Some gyms are doing away with contracts and at the same time reducing their monthly rates. If you're paying more for your gym membership than your friends then your gym sucks.