Make sure the mattress fits snugly -- if you can fit two fingers between the mattress and the side of the crib, your baby's head could become wedged there. A rolled blanket can be used to fill the space between the mattress and end boards. The mattress should be firm.
The slats of a crib should be less than 23/8 inches apart so your baby's head cannot get caught between them. Headboards should not have cutouts or decorations that could trap the head or neck.
All hinges and screws should be well set and out of reach, and there should be secure safety latches on the drop side that cannot be tripped, either by your baby or by any curious older children who may have access to the nursery.
All finishes should be smooth, and all paints should be nontoxic.
Don't use soft pillows or blankets that can become easily bunched; until infants can lift their heads high for long periods of time by themselves, suffocation when lying facedown in soft materials is a possibility.
Make sure you can lift a portable crib without too much effort, that it folds and stores easily, and that it is stable.