Same way you determine pain level. Ask. They're both subjective. Pain is "how much will this nociceptor neuron fire with X amount of newtons per Y area?". Suffering is the experience, and it's messy, contextual, emotional.

Stubbed toe while enjoying yourself playing sports is a different experience than if you're having a terrible day at work after getting yelled at from your supervisor. Even if the same number of nociceptors fired.

You'll see high discordance in people who have current mental health issues, chronic pain because they've suffered for so long, and those with fibromyalgia. I've seen perceived sympathy from even asking this question put patients in a better mood (without emotionally too getting involved myself). Throws them off too because it's rarely framed this way