@donajimena honestly we have so many guests that come and go its hard to keep track - the usual signs would be one person checks in while the other hangs back and kind of follows behind on the way up to the bedrooms, almost as if they don't want to check In together, as most couples tend to just come up to the actual desk together.
Similarly guests who book under weird, awkward names "Mr G" instead of a full name for example, always make me a bit suspicious especially if they're reluctant to fill out the personal info forms that ask for your post code, email and phone number etc.
Solo guests that check in then half an hour later you see someone else who hasn't checked in "sneakily" run past reception and enter their room, also suspicious 
We can't give out any personal info so if someone calls trying to catch their other half in the act by asking if they have stayed previously or are in house we cannot give them that info, I do take a note though and hand it to the guest in question and say "oh such and such called and asked if you were staying here, I couldn't confirm you were but you may want to give them a call" - always v satisfying to watch the guilty ones squirm when we do this hahaa.
Sorry my answer is so long!