Most gyms use bio-impedance scales to assess body composition changes over time, specifically muscle growth and fat loss. However, these scales have a significant margin of error, which makes them far from accurate, and that's why, in my opinion, they shouldn’t be used (at least not on their own) as an assessment tool. Instead there are a few indicators that you can assess and analyze together to get a pretty good idea of whether you’re gaining muscle or not.
1 - Gym performance: this is the most important one and it's why logging your training sessions is so important. Look at your rep strength across all sets for each exercise. If that’s trending up, it’s a very good indicator of muscle growth. Now, don't look just at your first set, look at all of them. For example, if you start at 70 kg for 10,9,8 reps and a few months later you’re doing 80 kg for 9, 9, 8, that’s a very good indicator of muscle growth. If instead, a few months later you're doing 75 kg for 9 + 70 kg for 9, 9, you might have gained some muscle, but there’s a possibility that most of your progress was in strength and not muscle growth.
2 - Scale weight: weight can fluctuate a lot from day to day, so measuring it just once per week (even on the same day) won't give you a solid insight on what's really happening. Instead, weigh yourself every day, first thing in the morning and after going to the bathroom. Then, take a weekly average (it's OK if you miss 1 or 2 days in the week). Do this every week and compare the weekly averages, assessing whether the weight is trending up or down. Here, we have two main scenarios:
If you're trying to gain muscle but have some body fat to lose and you're a beginner/early intermediate, aim to maintain your weight. You'll most likely recomp (gain muscle and lose fat at the same time, while the body weight stays relatively stable).
If you're trying to gain muscle, you’re lean(ish) and you're not a beginner, aim to slowly gain weight over time.
3 - Body circumferences: these are really good to assess progress in beginners with a lot of muscle mass to gain, and "skinny-fats" wanting to recomp. Take your arm (relaxed and flexed), chest, waist, abdominal, upper leg and calve circumferences, always in the same conditions (after weighing yourself is a good idea) and standardize each measure. When gaining muscle, you want the circumferences of your arms, chest and legs to increase, while waist and abdominal stay relatively stable. If you're an advanced lifter, there's no point in doing these, because the amount of muscle you have to gain is low, and this kind of measure is not sensitive enough. If you're a beginner/intermediate, you can take them every 4-6 weeks.
4️ - Progress pictures: the scale measures how much weight you gain; the pictures will show you the quality of it. Make sure to take pictures with good lighting and, if possible, always in the same place. This indicator is useful in two scenarios:
If you're a beginner/intermediate and/or you want to recomp, progress pictures can be used on the short-term (take them every 4-6 weeks, with the circumferences).
If you're advanced, progress pictures won't be really helpful in the short-term; use them only long-term (e.g. comparing two pictures at the same body weight, but years apart).
Besides these four indicators, you can also look at how your clothes are fitting. If your gaining muscle and not getting fat, your clothes might feel tighter around your arms, chest and upper legs, while staying the same in your abdominal area.
When possible use all the above indicators, so that you can better assess if you're actually gaining muscle. If most (or all) indicators line up, you're most likely getting bigger. So, for example, if your performance across all sets is going up and your weight and circumferences are also going up, this is a good sign that you're gaining muscle. On the other hand, if your progress is kind of stable, your weight and circumferences are slightly going up, and you look a little bit fatter on your pictures, you might be gaining too much fat, and you might need to make some changes to the initial plan.
Helms E, Morgan A, Valdez A. 2019. The muscle & strength pyramid: Nutrition (2nd ed). https://muscleandstrengthpyramids.com/