Wireless

You tell by checking your wireless router's user setup screens (see lietrature which came with router or download full manual from CD or maker's website).

Different routers display users-online in different ways but it should be pretty obvious as you explore the menus.

To deter intruders always enable wireless security.
 
 
WEP is not very secure -- though, frankly, how many of your neighbours do you imagine would spend the time to learn how to crack it ?

You can also invoke Access List (which filters by MAC ID -- only allowing computers with wireless adapters you recognise to connect). This is not particularly secure either but together would cause a potential hacker extra work.

May be better to replace the device which requires WEP as these are usually old and slow anyway.