Start with the main areas of the apartment, like the living room, dining room and kitchen. Rooms with the most traffic should be baby proofed first. If there are rooms you wish to remain off limits (particularly for adults without children) like a bathroom or bedroom, make sure you can close a door or install a baby gate. Then alert the parents of your wish to keep the room child-free. 
 

Are there any corners that could injure small heads? The edges/corners of coffee tables and some shelves may need rubber bumpers. 
 

Get down on all fours and search for objects that can easily be picked up and put inside little mouths. Knick knacks, pet toys and other items that are low to the ground should be placed out of reach. 
 

You may also consider installing baby-proof locks on cabinets that are low enough for young children to reach. This not only prevents a pot-and-pan orchestra, but also prevents kids from getting into cleaning solutions and other harmful chemicals. 
 

If the visiting child is very small and only coming over for a short while, you may ask the parents to bring a portable playpen so the child has a safe area to play without the need to take measures across the whole apartment.