The simplest trick for detecting old ARM emulation - ISTR was used on some Gameboy Advance copy protection: store a booby-trap instruction at PC+4 (i.e. the very next one). A real ARM has a pipeline that reads PC+8, while decoding PC+4 and while executing at PC. So the newly-stored instruction should have no effect. An emulator (which didn't emulate the hardware pipeline) would execute it.
edit: described in more detail here, among other emulation-busting measures from 2004 https://mgba.io//2014/12/28/classic-nes/