Early on in your discussions with the birth mother, you will need to come to a point where you and she have a preliminary understanding that she intends to place her child with you and that you intend to proceed with the adoption of that child. You should not be pushy about this. She will need some time before she comes to that point. Because you are undertaking both a financial and an emotional burden, however, in your conversation you want to be very clear about the fact that you cannot proceed to the next steps until this understanding is reached.
Perhaps the best way to put it is this: the birth mother needs to be as sure as she can be that she intends to proceed. That is to say, she knows she can change her mind and you also know this. But she must understand that it is not fair to you to allow you to poceed further unless she is as sure as she can be. She needs to say to you that she has had all her questions about you answered and that she is pleased with the decision to place her child with you. Do not go forward without that assurance and do not make the mistake of assuming al will be well. You do not want the birth motehr to say to you, "Oh, I never said I would place my child with you. I only said I'd think about it." She needs to say to you in some fairly clear statement, "I have decided that, barring some unforeseen circumstance, I will place my child with you for adoption. I feel good about this decision. I have decided."
You should understand that this does not mean you have a legal cause of action against her if she changes her mind. You cannot get your money back. In parental placement adoptions, you always understake a certain amount of risk. You merely need to keep that risk under control as much as possible.