My partner and I have never really had an issue with anyone changing us to doubles assuming we did not want one bed.

The only "issue" (and it was hardly an issue) that sticks out in my mind was at a Doubletree in Atlanta where the clerk became so visibly nervous he was flustered - seemed like he wanted to ask during the check in but probably did not want to offend us. We get our luggage out of the car and on our way back by the desk he then confirmed 'You guys wanted a king right?', we said 'sure did....thank you!'

When I worked front desk I just confirmed bed type like I did with every single guest I checked in and later trained my team to do the same. The simple phrase "I have you confirmed for a XXXXX for XXXXX nights." was sufficient to resolve any bedding discrepancies with any guest right then and there without any awkward/uncomfortable moment.