How to tell if bolt is SAE or metric?

Excuse me, could you please elaborate on how one can distinguish between an SAE bolt and a metric bolt? I'm having some difficulty identifying them, and I'd appreciate any insights you might have. Specifically, are there any visible markers or measurements that can help me make this determination? I understand that SAE and metric bolts have different sizing systems, but I'm not sure how to apply that knowledge in practice. Thank you for your time and assistance.