What is permitted to look and touch and what is not permitted while purchasing slave

From the Athar below: أنَّ ابنَ عمرَ كان يضعُ يدَهُ بيْنَ ثَديَيها ( يعنى الجاريةَ ) وعلى عُجُزِها من فوقِ الثيابِ ويكَشفُ عن ساقِها It tells that ibn Umar[ra] used to check the slavegirls in public in market. What is limit prescribed by scholars to check a slave before purchasing, meaning what it is permissible to see and touch as well as what is prohibited to see and touch