As a person who has been known to instigate prosecutions for H&S contraventions, the information given by the majority of those above is magnificent.
The people at the venue have the right to ask you to prove your equipment is sound and safe to the user and those who could be affected by it.
PAT is one way of helping to illustrate this.
I have come across rogue labels in the past at functions and have insisted that the equipment is either tested before use by a competent person or not used at all. THis can be devastating for the performer if this cannot be done, don't risk it.
In theory a "competent person" is better than a "Qualified Electrician" as there are incompetent electricians about. A competent person who is also a qualified electrician would be beneficial, however it is possible to be trained to use the PA Tester without being a qualified electrician.
To reiterate, get it tested by someone competent, as accidents do happen with faulty equipment. Also remember that even if the equipment has been tested it still can become dangerous and needs careful handling. Magic wands are not an option.