店員 and 社員: shop staff and company employees
Compare a role working in a shop with employment by a company.
A person working in a shop
店員 (てんいん) is a shop employee. 「店員にサイズを聞きました」 means asking a shop employee about size. It focuses on the person's role in the shop.
「店員を呼んでください」 asks someone to call a shop employee. It does not specify calling every employee by name.
An employee of a company
社員 (しゃいん) is a company employee. 「社員が会議に参加しました」 says that an employee joined the meeting. It does not specify whether they work in a shop or office.
「新しい社員を紹介します」 means introducing a new employee. Do not change the role into a customer or guest.
Two views of the same person
Someone employed by a company to work in its shop can be both 店員 and 社員. The words do not necessarily describe mutually exclusive forms of employment.
「社員全員が店員というわけではありません」 means that not every company employee works as shop staff. It is a partial negation allowing employees with other roles.
Does 「店員に確認してください」 mean checking with a customer?
Show the answer and why
No. It asks someone to check with a shop employee. Distinguish the role of the person being asked.