Identifying devices on my wireless network with question mark

Last night I checked the Netgear Genie connections, and had 24 items connected to the router.  Most had question marks (?) on the connections.  This morning, there are 19 connections on the network map, the only one identified is my Mac.  How do I identify the others?  Should I be concerned that others might be intruding on the wifi?  I have put seccurity in place with admin and password changes.  What should I do next to insure that security?