Bought foreclosure... what next?

[Misty Porter]

This will make the second foreclosure we have purchased. Our first one was unoccupied as the homeowner had died so we simply took possession the day of the deed being recorded in our names. With this foreclosure, it is a tax foreclosure and appears to be occupied though we have not seen anyone as of yet. While driving by,    we have observed a truck parked in the driveway and seen the front door open  indicating someone might be living there.   We have not finalized the purchase yet and we are wondering once we do... how to proceed? I know that if the owners have held over, as well as, if there are tenants or squatters, we will need to go the legal route to get them out or offer cash-for-keys. My question is more in how to proceed in determining if someone is in fact is living there. I’m thinking we should knock on the door after becoming the owners, but if no one answers, can we proceed to letting ourselves in and changing the locks? What if someone shows up later and says they live there? Any advice greatly appreciated!