How investers usually check mortgage rate?

[Chris Wang]

Hi BP,

I'm a new joiner recently from <the book on rental property investing>, I'm glad I can find a great forum to discuss property investment.
I do have a question about how investors usually check today's/this week's mortgage rate. I know a  couple of loan officers who are all friendly and responsive.  While I have the need to check the rates relatively frequently before making an offer because I like to get the numbers calculated roughly before making an offer, the issue is I don't want to bother the loan officers every day or every week checking the rates,  and then may not applying for a loan because of the deal is not ideal or offer not to get accepted. 

So I'm wondering if there is some self-serve method to check some realistic/practical rates in real-time. I know there would be bunches of results is I "google" mortgage rates, but I'm not sure which of them are reliable and practical.

Thank you, BP.