I'm aware of this question and read its answers, but I'm not sure if it specifically addresses what I want to ask, and also it'd be nice to have a more accessible answer since I don't know GR.
I'm reading a book on principles of relativity, and am going through the definitions of reference frames and free particles. From what I've understood from there and other answers on SE, a non-inertial reference frame is one in which a free particle is measured to be undergoing acceleration. This requires an observer to identify a free particle in the first place.
If I'm an isolated observer, there can be two scenarios:
I see a particle at rest w.r.t. me. How do I tell if that's a free particle without communicating with someone I know is in an inertial frame (and hence can confirm whether or not the particle is free)?
I see a particle that's accelerating: how do I know whether I'm in an accelerating frame, or if I'm in an inertial frame but that particle is accelerating?
As someone who started with the subject, it's very confusing for me. I'm not sure if in either scenario it'd be possible for me to confirm whether the particle is free or not.