That's a really big question. I used a lot of visualization, and trusted myself to know if I was really trusting in my choice to birth at home. I left it up to the day of the birth, and felt completely sure that morning. I also made it to 38 weeks exactly (my goal), and it was on the weekend, and the sun was shining (some small things that were important to me).
I had a perinatologist as well as a team of midwives for my care, and I asked every possible question I could think of to the peri, and ultimately decided to homebirth. I think it's important to change your relationship with your midwife at this point. I think that with midwives, they need to know EVERYTHING. They are there to monitor your physical, mental/emotional and even perhaps spiritual process in carrying these babies, and they need to know how you are feeling, and express their concerns as well.
My first birth was with my DD1, a planned homebirth, but my membranes ruptured at 35 weeks and labor didn't start for days. I was scared, and really hadn't established a completely honest relationship with my midwives, and ended up with a hospital birth with interventions that caused us much misery, and ultimately, I think were unnecessary. I am not great at the complete honesty part, but my midwife had a way of asking very direct questions. She was keeping an eye on my DD1 to see how she was doing with the prospect of mom delivering, and having twin sisters, as well as how my husband and I were doing together, as well as how each of us individually was doing. I felt very comfortable with her in the end, and really think that the way she communicated so openly really helped ensure that.
I'm a big heart and intuition person, and really did all the hoop jumping with the perinatologist to have the papers to show that everything was looking great, and I had proof (for all the people freaked out by a hb).