[Skullmeat]

To answer OPs question: It depends on the envrioment. If its a goverment owned library I wouldn't attempt it. Those are under heavy regulations and take breaches very seriously. If you want to go sniffing, your best bet is poorly secured places like coffee shops. If you are doing ARP poisoning, inteligence gathering is a must. MITM the whole network and most laptops will just crash or bog down, causing a denial of service. That would probably get thier attention. Do your best to identify potential targets and take them on one at a time. Another good idea is a long range antenna. The further away you are the less of a chance of being seen. If you must be on site, a raspberry pi is a great tool, since they are small and easily concealed.