Let's start by defining what you're after. Link speed and effective throughput are two different metrics, both...
quoted in Mbps. Link speed is the negotiated rate between a transmitter and receiver - for 802.11b, max link speed is 11 Mbps; for 802.11a and g, max link speed is 54 Mbps. Throughput is a measure of the actual payload being pushed over the link - for 802.11b, max throughput is around 5-6 Mbps; for 802.11a, it's around 26-27 Mbps.
You don't measure link speed -- but you can observe current link speed by looking at your adapter's client software. For example, using Cisco's Aironet Client Utility, choose Status and look for "Current Link Speed." Using Windows XP, just double-click on the wireless connection icon to view "Speed" on the Status panel.
To measure throughput, you need an application that pushes sustained traffic over the link and calculates an average over time. An easy way to get a rough idea of throughput is to launch a long-running FTP file transfer and average the throughputs displayed by the FTP client at the end of each transfer. Make sure the FTP server is on a nearby Ethernet with higher link speed so that wireless hop is the bottleneck. For more rigorous testing, use a performance benchmarking tool like NetPerf (zip) or QCheck (NetIQ). Note that you may need multiple stations to saturate the link. To learn more about performance testing, read this Atheros white paper on 802.11 benchmarking (pdf).