How can I tell if I have a stock recovery? I need to get rid of the yellow triangle in download mode on a Samsung Galaxy S3 (Android 4.3), for this I tried to use Triangle Away. It failed to reset the counter.

The app mentions that on some devices, both the ROM and the recovery must be stock. The ROM is but I do not know about the recovery. How can I check if it is stock? 

The one I have currently looks like this:



Click image for larger version
Based on the screenshot, you have the stock recovery.

The stock recovery normally shows "Android System Recovery &lt;3e>" whereas custom recoveries tend to have their name instead. There are two popular custom recoveries both of which look quite different to the one in your screenshot

ClockworkMod:



TWRP: