Number of abandoned (orphaned) blocks is a strong indicator of selfish mining activity. The entire idea behind selfish mining is for the selfish pool to outcompete the work of the honest pool. And this will leave behind a series of discarded blocks, where either the honest guy's work was wasted, or the miner took a slight risk and failed. Since every block race will leave such detritus behind in its wake, one could just count the number of such abandoned blocks to see if the rate is stable over time. A rise in the rate would indicate that a selfish mining pool is operating in the network.

The problem with this approach is that abandoned blocks are pruned inside the Bitcoin network, so it is very difficult to get a definitive count. A measurement tool that connects to the network from just one or a few vantage points may very well miss abandoned blocks, and it may erroneously give the impression that everything is fine when there are fierce battles being fought out inside the network. I am not sure how well blockchain.info is connected, but no web service can be everywhere at once to detect every single orphan, so I take its count of orphans with a large grain of salt.