I'd look for unusually large hardware purchases or development of novel computer processing to start. Any company or government which developed AGI would want to capitalize on it quickly by expanding its abilities or duplicating it to work on additional tasks.

So if NVDA gets bought out by TSLA and all processor development is utilized for in house projects, you might suspect TSLA has built an AGI and wants to secure the supply chain for new hardware. Or if META designs a new processing chip and licenses it to a manufacturer for its own exclusive use and buys gobs of it, you might think Facebook cracked AGI and needs more power to run it.

Governments might build AGI too, so look for unusual state hardware purchases or data center builds, plus highly successful policy decisions. For instance the USA able to neutralize terrorist cells they couldn't identify before, or China leapfrogging everyone in weapons development.

The first mover advantage will be huge since any AGI can be scaled up by speed and numbers, if not in some qualitative way. I disagree with Kurzweil's idea that it will take 16 years after AGI before reality breaks. There will be a hard takeoff by necessity because any time spent pondering the risks of expanding AGI is time that an unfriendly actor can build AGI and overpower you.