I'll give you the short answer. You can tell if a stutter is mild, moderate, or severe based on how it affects their quality of life or feeling of agency/independence. You can also measure it by how much anxiety and dread one feels before most speaking situations. If they feel like very rarely their speech gets in the way of or impedes development of career, personal relationships and such then that person likely has a mild stammer. If they feel like it's semi-impacting their relationships, career or life trajectory then they have a moderate stammer. So they know they could do better but they feel their communication skills are being held back by their speech. A severe stammerer will find performing basic things extremely difficult like ordering pizza on the phone, calling up friends, ordering at a drive thru, raising your hand in class. Moderate stammers can feel like this too. But severe stammerers will rarely have bouts of fluency, even when talking with close friends or family. I think in general, mild stutters will be more so repetitions than blocks. Moderate stutters will have prolonged speech blocks that coccur semi often. And severe stutters are blocks that are very prolonged and happen very often.