A few angles I would offer on this:
Most importantly, something is causing her to feel a level of distress. It may be ADHD, or it may be something else. If I recall correctly, your DD is a mid/late teen right now, which is a challenging age developmentally for anyone. So more than looking specifically for an ADHD evaluation, I would keep an open mind about what it is that currently makes her feel she is having difficulty regulating her attention/focus. For example, it may be emotional in basis, it may be physical/organic (which can also feed into emotional), or it may be neurocognitive (such as ADHD). Plus any/all of these may be amplified by the public health crisis that we all have been living through for the past year and a half. This requires a thoughtful intake interview with an experienced evaluator (e.g., neuropsychologist, child/adolescent psychiatrist), starting from what she has been experiencing over the years, what is bringing her concerns to the fore at this specific moment, and how she perceives her principal area of need.
I took the liberty of scanning some of your previous posts, and noticed that historically, not only has she been an exceptionally bright and compliant child, but she has also been reserved about her interior life, so it isn't that surprising that she may have been trying to manage this on her own so effectively that even her parents did not see signs of struggle. Please be gentle with yourselves!
This also may not be quite as expensive as you are anticipating, precisely because there are no academic concerns. Neuropsychological evaluations (which are typically the direction one goes especially when attempting to tease out a subtle diagnosis like ADHD in a gifted individual) often can be at least partially covered by insurance if you first obtain a referral/preapproval from your primary care provider. The main thing is to emphasize that it is -not- an academic concern, but a medical concern, such as with recently-reported changes in her ability to focus, or feelings of anxiety regarding meeting her own expectations, or whatever the actual symptoms she is reporting are. Insurance companies typically will -not- pay for learning disability evaluations (because public school districts are responsible for those, for any child who resides in-district, even if they attend another school).
Alternatively, you could start from the school district, and request an initial evaluation, with concerns regarding regulating attention, and perhaps executive functions/organization (these are typical ADHD concerns). Depending on your state, the district may or may not be able to decline to evaluate; in some states, the bar is fairly high for rejecting parental requests, while in others, districts routinely reject requests when the student is performing at or above average. If the district evaluates, this would be at no cost to you beyond whatever you are already paying in taxes. Keep in mind, however, that the client in this case is actually the district, whereas in a clinic-based eval, you are the client. Most school-based practitioners do, of course, view the child as their primary client, ethically, but they may experience institutional factors that affect what they are able to do, as well as resource limitations for some of the pricier computer-based assessments. But a good school-based psychologist can generate an evaluation (comparable but not entirely identical) on a par with that of a good clinic-based neuropsychologist. (Full disclosure: I'm a school-based evaluator.)
And with regard to the difficulty of identifying a well-compensated disorder, my thoughts would be that, first, there is a point at which a neurocognitive profile is sufficiently well-compensated that it really isn't pathological any more. That point is generally when it doesn't interfere with major life functions, and it doesn't cause distress to the individual. It appears that whatever is going on doesn't, on the surface, interfere with major life functions, but that it is actively causing distress.
Secondly, a good neuropsychological evaluation will involve objective performance measures that are designed to isolate specific neurocognitive functions as much as possible, to reduce the confounding effect of compensatory strategies (or interference from cross-domain weaknesses). For ADHD, there are a number of computer-based tools that have a pretty good track record in this regard. In addition, a skilled evaluator will look for interpretive patterns across multiple instruments, both computer-based and in other formats, and may be able to pick up on subtle effects.
Mainly, it will be important that the evaluator have a strong rapport with your DC, and experience with how gifted young people mask or compensate for their challenges. And an open mind.
