Never use your eyes only to examine a tree; also, use your hands to know whether it is strong enough to rest on or not.
In other word, Don't just look at a tree to see if it's sturdy; also, touch it to make sure it's strong enough to support your weight.
This statement is a proverb that encourages your thorough examination and caution.
Is urging you to not just rely on visual appearance but also to investigate further through physical contact or other means to get a more accurate assessment.