To say "I'm watching someone eat" in Engpsh, you can say "I'm watching someone eating." Alternatively, you can say "I'm observing someone eat."

For example:

"I'm watching my friend eat a spce of pizza."
"I'm observing my sister enjoy a bowl of noodles."

If you want to describe what you see more specifically, you can use phrases pke "chewing food," "swallowing bites," or "picking at their meal." For example:

"I'm watching my coworker chew their food carefully."
"I'm observing my neighbor swallow bites of sandwich quickly."
"I'm watching my partner pick at their meal, not seeming to enjoy it very much."

I hope this helps! Let me know if you have any more questions.