Read Status

I have been away from the hobby for a bit. How can I determine if and when one of my PMs has been read? I have noticed that sometimes before opening a msg to me there is a receipt notice to accept.

That info is usually at the top on your Inbox, just below the drop down box that lets you check your Sent folder. You should see something similar to what's below and when you click it, it will take you to the lists showing who did & didn't confirm receipt.

You have 145 confirmed and 52 unconfirmed private message receipts.