Know who you're dealing with/ knows exactly who he's dealing with
To know clearly what kind of person you are dealing with, what to be careful about, and what you can expect from them.
(Modern Hebrew)
Translation
Expression (Hebrew)
יָדַע עַם מִי יֵשׁ לוֹ עֵסֶק
Definition (Hebrew)
יֶדַע בְּבֵירוּר עִם אֵיזֶה סוּג אָדָם הוּא בְּקֶשֶׁר וּמִמָּה צָרִיךְ לְהִיזָּהֵר אוֹ מָה אֶפְשָׁר לְצַפּוֹת מִמֶּנּוּ.
(הָעִבְרִית הַחֲדָשָׁה)