If they want you to not call the agent on the sign, it's a scam.
If they want you to send money in exchange for keys, it's a scam.
If they are out of the country doing charitable deeds and just want someone to care for the home, it's a scam.
If they are dramatically under the market for similar homes, it's a scam.
If the whole thing is just too good to be true, it's a scam.