A network discovery tool is a good place to start. Typically it is an inexpensive application (perhaps, even free) that works with your built-in 802.11 wireless adapter. It is easy to use and provides basic information about local access points and wireless routers. An 802.11 network discovery tool will report the Service Set Identifier (SSID) for each access point (AP) it detects, along with the channel used by the AP. Approximately every 100 mSec an AP transmits a small beacon packet and a discovery tool (running on your laptop and using its 802.11 wireless adapter) detects the beacon and adds the packet information (including the AP's SSID) to its list of known access points. In addition, the discovery utility may report signal strength (in dBm units) of the beacon as detected by the client adapter. The beacon's signal strength is an indication of how close the AP is to your current location. Though this is useful information, it does not tell you anything about non-802.11 devices or even how busy the access points are. That is, your laptop could be sitting next to a microwave oven and the discovery tool would be clueless as to its existence. The network discovery tool only knows about beacon packets transmitted by 802.11 devices and can not see non-802.11 transmissions.
A network discovery tool reports all access points -- both those that belong to you and your neighbors. For troubleshooting purposes, one is primarily concerned with your APs -- in particular, the signal strengths of their beacons (often reported as RSSI). When this value is above a certain threshold, e.g. -75 dBm, then the AP and client adapter are close enough to communicate. Other than reporting that the AP and client adapter are within proximity of one another for communication purposes, there is not much more the network discovery tool can provide in terms of helping you troubleshoot RF-related problems. Though network discovery tools may also report which channels are used by neighboring APs, this is not nearly as important an issue as RF interference caused by non-802.11 devices. Since network discovery tools can only see neighboring 802.11 APs, then they are of little use in detecting non-802.11 wireless devices.
A common misconception is that AP beacon strength is a measure of performance -- it is not. The signal strength reported by a network discovery tool is the signal strength of a beacon as measured by the 802.11 wireless adapter installed on your laptop or desktop machine. The signal strength of the beacon is a reflection of how close that AP is located to you -- and that's all. It is not a predictor of the performance or throughput available on that particular wireless network -- it is simply an indication of the AP's physical location relative to you.
If the AP with the strongest beacon has 24 client adapters associated with it that are actively transmitting and receiving information, and if you connect with that AP then you will be client number 25 and your network connection will seem slow. On the other hand, if you instead choose to associate with an AP whose beacon strength is weaker but which is not associated with other client adapters, then you will likely experience better performance. Furthermore, the AP with the strongest beacon signal may be using a channel that is subject to RF interference -- again, degrading its performance. Though a beacon's signal strength can affect it's performance, what's more important is the number of client stations that are competing for the same AP and whether the channel currently used by the AP is subject to RF interference from other wireless devices in the vicinity.
(1) Let's say you are at the airport and wish to connect to a WiFi network. The network discovery tool lists all the APs (and wireless networks) in your vicinity. Based on signal strength, security settings, etc. you'll choose one from the list to connect to.
(2) You are tasked with installing a wireless network and wish to determine whether the 2-dimensional (or 3-D) area covered by an AP can actually reach all corners of the area to be serviced. When a WiFi network is first installed, the location of the AP and the type of antennas used and their orientation are most important. With a network discovery tool installed on your laptop, you'll move around the area and note the signal strength of the beacon emitted by the AP at different locations. Ideally, the beacon signal strength should be stronger than -70 dBm at all points. If not, you'll need to experiment placing the AP at another location, re-orienting its antennas, or perhaps decide to add additional APs.
Related to network discovery is a more structured approach referred to as a site survey. A site survey is an application that also uses your built-in 802.11 wireless adapter. The software differs from network discovery in that it is usually more expensive and the end result is a 2-dimensional color map of your location, where colors indicate beacon strength. That is, the resulting coverage map provides a graphical representation of beacon strengths and the geographical area to which they extend. The site survey is performed by loading a copy of your floor plan into the application and then walking the premises -- using either GPS or clicking on the floor plan your current location is noted and the beacon strength recorded. The goal, as in network discovery, is to ensure the area to be serviced can be reached by the beacon from an AP. As with network discovery, the results from a site survey are not an indication of the performance you would expect from the wireless network. Performance is affected by RF interference and client load, neither of which is measured by a network discovery tool or by performing a site survey.