For me I can tell I’m having an anxiety attack or a panic attack by the sensations in my body. A meltdown for me usually feels more instant or suddenly overwhelming whereas a panic or anxiety attack the first sign I have is that my hands starts feel bigger than they actually are and my brain fog gets very bad. Where as a meltdown usually starts with agitation and more aggressive of a reaction internally. I don’t know if this helps but that is my experience.

First of all, alike, that response was everything I was struggling to verbalize. To offer my autistic two cents- autistic people take in way more information from the world at large than neurotypical people. As a result, a lot of us struggle with various forms of processing (audio, visual, etc.) when our nervous system can't take in and process any more external information, our brains become overwhelmed. For me, it feels like I'm suddenly surrounded by a whirling tornado of sandpaper, screaming in my ears. My vocal cords lock, and all I can do is keen and wail. I lose sight of my surroundings, and all my muscles contact. Standing is physically impossible. It takes every ounce of cognizant thought I have to keep myself from hurting myself. I'm fortunate in that the worst times have been at home, and my husband has developed a routine of putting a tv in whatever room I'm in, putting on my most calming show, leaving me my soft fuzzy pj's, a drink, and a snack, before quietly waiting nearby, out of sight. But every autistic person experiences their world differently. That's what my meltdowns feel like as a result of more inputs than my system can process. It's kind of like repeatedly sticking a branch in your bike tire until the spikes break. Anxiety and panic attacks happen as a result of a perceived fear of that. Ie- the thought of grocery shopping makes me anxious and gives me panic attacks. The act of shopping is such an assault on my nervous system, that it causes my system to become overwhelmed leading to totally restaurant collapse.

Meltdown from my experience has a longer warning period before the inevitable reaction. It is a different experience for other people though.