The screws, bolts, and/or other fasteners should all be in place. Are they loose? Will the position of the mattress hold under your baby's weight? It is imperative that this be tested before you put your baby in the crib. Take something that approximates the weight of your baby at about 4 months old. Bounce it off the mattress to be certain it will hold its position. If it doesn't it could cause serious injury to your baby or worse.
When setting up the nursery (if it isn't already up) consider very carefully where you will place the crib. If you place the crib near a window and you have Venetian blinds, either shorten the cords or anchor them somewhere that your baby can't reach and get a hold of them. If it all possible avoid placing the crib near the window. As your baby grows into a toddler and s/he attempts to climb out of the crib; s/he could possibly fall which could cause serious injury to your baby.
Blankets and your baby are not necessarily a good mix. With the incidents of SIDS today, you want to be sure to do everything that could possibly put your infant at risk. It is more advisable to put your baby to bed in a sleeper. If you absolutely have to have a blanket on the baby, tuck it tightly around and under the foot of the mattress with your baby's feet touching the footboard. Also you don't want to put the blanket any higher on the baby than up to his chest as that will help prevent him from slipping under the blankets and suffocating.
Bumper pads are a great concept but unless they are secured properly, there is a risk of your baby slipping between the mattress and the bumpers and possibly suffocating. If you use them they should be anchored in at least eight places one at each corner of the crib and at least two spaced evenly on each of the sides. There should be a total of 16 ties in all, for the top and bottom.
Mobiles are a nice addition and look adorable but... the caution here is that if you use a mobile as soon as your baby starts to sit up on his own the mobile should be taken down to prevent your baby from getting tangled in it. Also make sure that it has no small removable parts that your baby could choke on.
If your crib is second hand no matter whether you had for a previous child or you got it from someone else check out the mattress carefully. Make sure there are no cracks or holes in the mattress covering. Make sure too that the mattress properly fits in the crib. Here again, your child could slip between the mattress and the sidebars or the end boards and suffocate. The mattress should fit snugly in the crib. Now the sheets you use in your baby's crib should also fit properly and not slip and slide. Sheet anchors are available that hook on the sheet under the mattress and keep it in place.
The position of the mattress is imperative for your child's safety. Most parents put the mattress at the highest position when the baby first comes home because it is so much easier to change him in that position. As your baby becomes more active you will want to lower the mattress accordingly. Once your baby is able to pull up to a standing position put the mattress in the lowest possible position and to be sure your baby is safe, measure the distance between the top of the side bar and the mattress. In the lowest position the distance of the top of the side bar should be no more than 26 inches above the mattress. If your child's head is over the side bar or they climb out of the crib, it maybe time to move your child to a regular bed. Some cribs are convertible into beds tat will grow with your child.
The crib itself should be looked over for things that might put your baby at risk. Have you seen those cribs that have ornate designs carved into the end boards? They are beautiful but they pose a danger to your child. Your child could get his head or arm and leg caught and sustain an injury. The simpler the design of the crib the safer your child may be.
Since approximately 1974 federal safety guidelines for cribs state that the slats should be no more than 2 3/8 inches apart. This is to prevent your baby from getting his head stuck between the slats. This could cause injury to your baby but it would necessitate the removal of some of the slats and that alone would compromise your baby's safety.