If you are listed as a dependent on the insurance, you can use it. But you should read the policy documents in advance so you know what kind of money you would need to pay. While copays are less than what you'd be paying out of pocket, the policy often dictates where you can go for services. Like my policy's in-network lab work provider is LabCorp so every time I have blood taken, it has to be done at a LabCorp location in order to minimize my out of pocket expenses. Going out of network, even inadvertently, could cost you the full price of the bill based on your policy's coverage levels.
You should also know that every time you use your insurance, it generates a claim. The primary person on the account will be notified that they have an Explanation of Benefits available usually a few days after that. It details what kind of service was done, where it was done, how it was coded, and the breakdown of what insurance paid and what your expected contribution is. I get these via email, but I know my mom has them mailed to her. So your parents would likely know within a week or two that you are using their plan, where you used it, and vaguely what you used it for.
I lived independently of my parents from the age of 17 on. Paid my own rent, did my own taxes, had bills in my name, etc. I was still able to be listed as a dependent on my mother's health insurance until I turned 26 last year and had to get my own policy. I am not sure if they can legally drop you from their policy before you turn 26 if you don't have a policy of your own, but they may be able to.
I'm also not sure if this is universal, it may just be for ACA plans, but I know with mine, I can only make changes to the policy if there is a change in life circumstance (ie. I get married and I need to add my wife to my policy, or my wife and I have a child and I need to add the child), OR it's during a specific time of the year (Open Enrollment season). So if that is a universal thing, you could find out when their Open Enrollment period is and start using their insurance immediately after that window is over. It would basically give you a full year to use the policy unless they find some way to boot you off of it or otherwise restrict your access to it.
