Implicit/Tacit mechanisms in Bitcoin Core disclosing Selfish Mining
It is impossible to detect a drop in hashrate working on a chain because foreign hashpower is only evident in the discovery speed of blocks, except if it were deducted from a pool publicly sharing its mining stats. Thus the dropped hashrate would only be evident over sufficient long spans of time.

The only indicator of selfish mining would be the actual publishing of a previously unknown longer chain. I believe I've read somewhere that this will be shown in BitcoinCore with a "Reorganization" warning.