Definitely determining an SD card's speed or transfer rate is impossible because transfer rates to solid state, or "flash," memory always vary. SD cards are grouped into speed classes that guarantee they will achieve at least a minimum speed. This minimum speed is important for applications such as video recording that require a lot of information to move quickly. There are programs that can determine an SD card's transfer rate, but you should run the tests several time and average the results for an accurate idea of your card's speed.
Download Nodesoft's Disk Bench (see References), which works with anything your computer recognises as a drive. It sends a small string of information from your hard drive to the SD card and precisely measures the amount of time it takes to get there. Click the "Create File" tab at the top of the window, then click the ellipsis ("...") button and use the window that appears to navigate to your SD card. Click the "Start Bench" button and wait for results.
Download Steel Bytes' HD_Speed (see References), which can deliver SD card speed test results without installing anything. Instead, it runs from a tiny, 80 KB application that is available in a variety of languages. It even displays transfer speed in a real-time graph. Select your SD card from the "Drive" drop down menu, then select a test file size if you wish (the 1MB default should be fine), and then click "Test."
Use Crystal Mark's Crystal Disk Mark (see References) to test both the reading and writing speed of your SD card. Like HD_Speed, this program runs from an application without installation. The program is free to download and works well for a basic transfer rate tester. Select a number of test runs, a file size and your SD card from the drop-down menus across the top of the screen, then click the "All" button. The program automatically averages the results of each test.