I would never just take a personal check. I would not risk the relationship of a close friend or family member, and would not take on the risk from a stranger.
Treat your lender like they are a bank - because they are to you! Make sure they are protected at all times, and will not lose any money. I assume you would like for them to loan you money again, yes? So make sure they get paid, even if you don't.
Close at a title company. If this a new purchase, then I would assume you are closing at one anyway. Have them send the money to title. If it is a cash out refinance, close at a title company. Get them a lender's title policy, along with hazard insurance naming them as the mortgagee or loss payee. Get an attorney, have them draft the loan docs to the terms that you have agreed to. Send the loan docs to the lender for review, along with the title commitment, and a copy of the settlement statement for review prior to closing. Maybe they won't care, but guess what, you just became a whole lot more credible to them. Post closing, make sure you have the title company send them all of the docs in an email format as well as the originals. Don't forget to CC your self. When the time comes to pay them off, be prepared to provide them with a release of lien.
Have you thought about how you are going to keep track of the payments made and interest owed? Is there an amortization, or interest only. If it is amortized, that can be a pain to self service the loan on your lender behalf.
I applaud you for asking the question in the beginning. I feel like I have been a bit of a rant, and I am not meaning to overwhelm you. But there are some serious items that need to be addressed. Let's look at it from the other side. Would you lend me money for my real estate business by just writing me a personal check? (please say yes!) Would you like to know that your money was protected? Would you like to know how I was planning on paying you, and when and how I would pay it all back?
You treat your private lender right, and they will treat you right. (and probably tell their friends about you)
Good Luck