These mini concerts give you a chance to hear the band or dj live, rather than via Internet. Most of the band/dj websites list their showcases - if you don't see any offered, don't be afraid to call and ask. However, try to request a dvd or other recording of the entertainment performing at an actual wedding or similar event. The showcase is typically in a small room with a limited amount of people, just a bit different than a large venue with 100+ guests, and you want the opportunity to see them work a crowd. If possible, ask to stop by a wedding or other event that the band/dj is actually playing at. A lot of companies offer this service-they just have to run it by the wedding couple first.
Some questions to ask during a band/dj showcase:
Does the lighting scheme (shown at showcases) come with the package?
Does the package include ceremony and cocktail house music?
Is the band able to be modified (i.e. extra horns, singers, etc.) and at what cost?
Can you see a song list?
Is there a preference of music the band leans toward playing?
Does the band/dj require compensation for travel, overnight accommodations?
How does the band/dj plan to work with other event staff?
What is the band's/dj's plan of action in moderating the night in terms of timing, meal service, etc.?
Can they provide references that you can speak with?
What happens if one of the band members can't perform on the day of your wedding?
How does the band/dj cater to audiences comprised of people young and old?