Most wireless PC cards are supplied with some kind of wireless card utility that displays SNR. Unfortunately, such utilities are device-dependent and won't work with just any kind of PC card.
To observe SNR in a device-independent fashion, use an external receiver. For example, you can put the PC card in peer-to-peer mode and run NetStumbler on another wireless-capable laptop near the station. No matter what kind of 802.11b/g PC card the station is using, NetStumbler will display that station's SNR, MAC address, and other values. Note that NetStumbler doesn't show stations in infrastructure mode. Most wireless LAN analyzers can also display SNR for nearby transmitters; consult this page for a list of commercial and shareware tools.
This was first published in April 2004