https://interlockkit.com/

You have two bus bars in your breaker panel. It is designed to be fed from one main breaker. But, electricity can flow backwards through a circuit.

An interlock kit is a simple device that allows either your main breaker to be turned on, or a different breaker to be turned on. Not both at the same time. With such an interlock in place, you wire the auxiliary breaker to a generator inlet box, so you can plug your house into your generator.

A non-contact voltage detector is the rest of your answer. The detector is locked on, and placed where it can pick up voltage on the main feed into your house: tape it to the main feed into the breaker panel. When power is restored, it starts beeping, and you know you can transfer back.

Do it the right way, because you aren't the only person who might be doing it, and the people who learn from you might not fully grasp the concepts involved. They might get lazy, skip steps, and set their home and/or generator on fire.

This is what automatic generators are for. They come on after sensing the power drop out and when it comes back on, it shuts down automatically. I have such a system because I live in the middle of a forest and trees fall over sometimes. And for bonus points, it can happen when the outside temps are at -30F/-29C and be off for a few hours or days.

It is expensive, but worth every penny.

My house has totally fucked-up electrical. For some reason, there's a secondary breaker box in the basement which switches some of the outlets and lights in the house over to running off the generator outside. So when the line power comes back on, the lights that aren't switched to the gen come back on (if I happened to have them on when the power went off, at least).

I've lived in this house for 15 years now and I've been without power here for a total of seven weeks during that time. I like how many trees we have here but that's the price we pay.