I believe you need to know the person. I don’t see any other way because panic attacks are represented in different forms

I actually just had a conversation about this with a friend! From what we could tell experience-wise, the main difference is that meltdowns lack a visceral sense of impending death. I am incredibly overwhelmed during meltdowns and feel an intense general fear, but I've never been convinced that I was dying during one. My friend has a panic disorder and was told that one of most identifiable symptoms of a panic attack is the belief that you might be dying. From a psychological standpoint, the main difference is that panic attacks are a hormonal over-reaction to initiate intense response to danger, while a meltdown is a hormonal response to intense emotions that autistic brains are unable to process and/or regulate. TL;DR Both are extreme responses to stress, but panic attacks are a disproportionate reaction to a stimulus that the individual's brain should reasonably be able to handle, while meltdowns are a loss of control caused by an experience the autistic brain is resonably unable to handle.