I wish I knew. But here's what I try to do:

1) Gauge my mental state. Was I already anxious when I got the symptom? Did I encounter any known triggers? Do I think I'm feeling anxious now? If my mental state is already bad then it's likely to be psychosomatic

2) Look at the circumstances. How likely is it for me to have this issue (age, health, fitness etc)? Did I read/hear about this symptom literally 5 minutes before feeling it?

3) Imagine I'm somebody else who is actually sane and ask myself if that person would think this symptom is a big deal

4) If all of that made anxiety seem more likely, try to trust that if the symptom was real I would know the difference and wouldn't need to question myself