This was a suggestion or piece of advice to someone to express his openion about somebody.
'Tell' doesn't mean 'say' here.
"You can tell" is a very common idiom meaning, "you can see/perceive/notice ...."
"You can tell he is a wearing a toupee."
"You could tell he didn't know much English."
"I think you'd be able to tell if I didn't have a bath for a week."