Depends. Amateur/cheap installations stick out like a sore thumb. Professional installations will be almost undetectable. . If you don't want to look for them, keep your eyes open for a nearby room that they won't let you in. The cameras often use wireless transmission, but the range is limited. Without knowing exactly what they are using, the equipment to detect it is very expensive and requires training in it's use. Can you take a laptop into the environment? You can download NetStumbler before you go there, then run it and see if there are any active wireless networks in the area. If you see one, then we know to look for a 802.11a/b/g/n transmitter. If not, then they are using something more clever.