Subject To home for sale by owner

[David A.]

Hello everyone,
Yesterday I saw a fsbo sign next street over to my parents house and decided to call out of curiosity. I spoke with the owner and asked him a few questions. The house is 2,300 sf 3/2/2 2 story and he wants $136k. I asked him if he would be willing to seller finance it with my limited knowledge on the subject. He said he wouldn't mind. His payments are $900/mo but I am not sure if this is PITI. I could get $1400 in rent for this home. I continued making conversation with him, he is a disabled vet and wants to move to a bigger house because his mother in law just moved in with them. I may be wrong but imho I think he wants to downsize or get out of his mortgage and not willing to be a landlord. The question I have is when is buying a house subject to a good deal? The house was built in 2006 so he has about 6 years in it. Is there a way to find out how much equity someone has in their house or only by asking. In my past bandit sign marketing I visited a few homes where they were going through divorce or moving and wanted to get rid of the house asap. I passed these deals to another investor who I thought I could trust but that is that maybe he took the house subject to and owner financed it himself. I read somewhere that there is a due on sale clause where the mortgage company can call the loan due immediately if the house changes owner but I'm not too sure. Any guidance or advice would be awesome. btw sorry about the runon sentences I have a crying baby and typing on the ipad lol.