Else you'll need a sack-truck, somewhere near the dancefloor...

(sorry, that was low...even from me)
As recommended above definately contact the bride and groom for this info...
If you're nervous about talking to the on the phone or face-to-face before the big day...and are good on "Microsoft WORD"...draw up a letter or form. You know a sort of confirmation of the time, date and venue (OH AND THE AGREED FEE

) etc...this will assure them...as will putting your contact details on there too, with a "Dont hesitate to contact me..."
Then, have a tear off bottom bit to the form with questions like "Bride & Grooms choice of first dance:___________________", will both sets (4 people) of parent be there?_________________. "are there any other anniversaries/occasions etc which you would like mentioned?___________.
Send this out at least a couple of months
If you havent got, or cant find ANYWHERE, a copy of the Bride & Grooms choosen tune, and its getting within a fortnight of the big day....call them and ask if they have a copy...or an alterative.
Also, on the night (of the big day) I tend to "interview" the Bride & Groom early on (off-air) and ask them what time they'd like the first dance to happen (they ALWAYS say "uhhh what time is it now?") - they might be waiting for a special evening guest to still arrive etc, who wants to see the first dance, I also ask things like:
Would you like background music for a few minutes during the buffet so people can have a good natter about ol' times while munching?
Was there a stag and a hen night (saves you calling "all those who attended the Stag night" up to the dance floor, when there might not have been on...doh!

)
Where are they honeymooning?....(you might have a record thats apt to play)
Are the bride & groom staying until the end? Many a wedding reception sees the bride & groom disappear off in a booby-trapped car, or a taxi with a nervous looking taxi...about 30 minutes before the scheduled end of the night...be prepared.
If leaving early (see directly above) would the bride & groom like an Arch (two lines of guests which the happy couple wander down toward the exit...as you play "Goodbyeee" or the "eastenders singalong album" or "she's having my baby"

(RISKY!!!!)
Have fun....