To find your maintenance calories you can use an online calculator like this one, and then test it's accuracy for a few weeks. All that is needed to test is basic math skills and *consistency*.

The first step is to make sure you weigh yourself once a week at the same time of day and under the same conditions. Then you would eat the same amount of calories every single day for a week, and at the end of the week weigh yourself again and note any changes in body weight (this may take multiple weeks to get a more accurate result since body weight sometimes fluctuates naturally).

It is important to note that a pound of fat is roughly 3500 calories. Meaning if you ate 500 calories less than your maintenance every day of the week, you'd lose a pound by the end of the week (500 * 7 = 3500).

For example: Let's say that your estimated maintenance calories are 2500 calories per day. If you eat 2000 calories every single day, this would put you in a 500 calorie deficit each day of the week. If you were to weigh yourself once a week, you should see a one pound decrease. If, for some reason, you see that you only lost 1/2 a pound, then you know that you're actually consuming only 250 calories below your maintenance, rather than 500. This means either you're miscalculating your calories that you're eating or your maintenance is slightly less than what is being calculated. From here, you can adjust your diet accordingly by removing another 250 calories to reach a 500 calorie deficit and lose 1 pound per week.

Remember, it's not important to be EXACT; as long as you're estimating closely and being consistent, you will make progress.