Some DJ/MCs invite clients to see them at other receptions whilst they are performing. However, most DJs refuse to do this. Two problems with “visiting” is the fact that the reception you see is created specifically for an entirely different group than you and your guests. It is a significant imposition to invite strangers to someone elses wedding. Most clients would not like to have uninvited guests turn up at their wedding, especially where it is to benefit the DJ/MC they are paying.
The best way to judge a prospective D/MCJ is an in-person meeting which will help you to determine if you will be comfortable with them. This allows you to judge his/her appearance, posture, manners, speaking ability, and knowledge of receptions and music. A professional DJ should be friendly, personable, and easy to talk with. He should actively listen to your concerns and address them in a positive manner. He also should be able to explain the basic order of events, and what his strategy is for making your reception unique and special. A true professional is flexible and will always have more than one way to accomplish a goal.