Hi,
And the answer is... yes and no, more towards no. Some programs (including some web browsers and web apps) can tell if you've got an assistive technology such as a screen reader running by querying specific information from the host system (the computer the app runs on). If the operating system (Windows, for example) informs the app that a screen reader is running, the app can either handle things differently by itself or expose this fact for others to use (technically, this is called "consuming"). You can in fact tell NVDA to not set screen reader flag when it starts, which "hides" its presence - there are other ways to detect if a screen reader is running, and one such way is via Windows Registry (won't go into details here); this means apps looking for a screen reader will find that no-one is responding to such a question when in fact the screen reader is moving around without identifying itself (some apps can probe specific places in the operating system, but it runs into performance problems; I can discuss details if you really want to know how a system call works inside an operating system). Therefore, the answer is really no.
Cheers,
Joseph