I put beer traps round my newly planted out sunflowers - couldn't believe how many slugs were in there 2 days later! and the seedlings completely untouched - amazing. However, it is more difficult to use them in a greenhouse for obvious reasons. I had a snail infiltrate my greenhouse the other day but luckily I managed to find it and eject it, but not before it had chomped down a couple of brocolli seedlings.
Have a good look round under pots and staging etc regularly to make sure there aren't any hiding and use slug pellets as a last resort. They can't do too much damage in a greenhouse can they?