In order to check the MOS has been sent and received by the vendor and buyer, you just have to look at the relevant contacts timeline. The quickest way to do this from the memorandum of sale, is to directly click on the contact name at the top of the page which will take you directly to the contact profile.
From here when you go to the timeline, you can see if the MOS has been sent, and if its been opened, and at what specific date and time.