I don't think that there is usually an assessment unless you are joining a gym specifically designed or marketed for weight loss (Curves, LAWL centers, come to mind). A more generalized gym might offer that service (mine will do caliper measurements and some stress measurements if you wish as an extra service- with an extra price).
Any good gym will be willing to have an employee give you a tour. In fact, I would recommend that you DON'T join the gym until you have had a thorough tour and run-down on all of the services that they offer. My gym also offers a free introduction to each of the machines and weights (and/or training session) by one of their trainers within the first month of your membership. I didn't do this, but I regularly see new people getting help from a trainer, and it looks like they get very good instruction.
There are usually predictable busy and slow times at my gym. The busy time being before noon on the weekends and between 3:30 and 6:30 in the evenings during the week. For me, this means that if I go then I might have to wait for a cardio machine (they seem much more popular), but people don't generally wait and watch, they just go work on something else until a machine gets free. There is a waiting list, but I've never used it.
I find the equipment very easy to opporate- though not always easy to live through.
The cardio machines work pretty much like the ones they sell to homes. The nautilus machines are easy to figure out, even without instruction (though, like I said, free instruction is offered at my gym). There are pictures that show you how to use them, and as long as you don't load on the weights until you've "practiced" with a new machine, they are very safe. I have to admit that I'm still a bit intimidated with the free weights, but I know I'll get that going with time.
I really love my gym, and I feel that I get such a better use of my exercise time then I would exercising at home. My main advice would be that, if you have more than 1 gym in your area, try them all out before deciding which one to join. Gyms can have very different atmospheres, and you'll be happier at the "right" gym and more likely to go on a regular basis. (For example, I don't like to be bothered with "salesmen" when at a gym. My gym never tries to sell anything-suppliments, training, or classes. They are all available for those who want them, but no one will come up to you and pester/encourage you to buy. Another gym in town is just the opposite, and though it is popular, I wouldn't care to go there.
Good luck! I'm sure you'll have a great time!