The three lawful bases
Explicit consent. Condition-of-entry (they can decline, you can refuse them entry). Support of a lawful police search on premises.
The mechanics of a professional bag search
Ask the customer to open the bag and reveal contents to you. Do not put your hands in unless they consent to that specifically. Keep another guard within visual range.
What to do when you find something
Prohibited item on premises rules: request removal from site, retain evidence if criminal, call police for indictable-offence contraband, document immediately.