Kashrus in home establishments | How do you know if you're really eating Kosher?

If you make products at home and sell them, do you need a Hechsher?
Why is it different than eating at someone else’s house that you don’t need a Hechsher?
Is this Hechsher different than a Hechsher on a food establishment?
How can you do spot checks in a person’s private home?
There are hundreds of Hechsherim – How do you know what you really can rely on?
What standards does a Kashrus need to have to really rely on them?