How can I determine the SD card's path?

I'm on a Thinkpad with Android 4.0.2 trying to open an HTML file on an SD card, using the default browser. Answers here have indicated you should access file:///mnt/sdcard, file:///mnt/sdcard-ext, file:///sdcard, and many other combinations, but none of them work for me: I just get "Webpage not available". Apparently different machines mount the SD card to a different path. How can I find out which path my SD card is mounted to? Or if that's not the problem, what is?