So here is the top ten. Feel free to leave it lying around for those in the "exclusive and serious" phase.
1. Dogs or cats. Are you both dog people or cat people (yawn, yes we know), but as importantly, how are you with pets: sentimental and oversensitive on their behalf? Kind and responsible? Flakey? Better be on the same page because this is your parenting preview.
2. Mooching. Are you going to all the galleries and the artefacts-of-war exhibition and the gourd-whittling demonstration, or are you basically drifting while keeping an eye out for a good lunch place?
3. Music. Taste in, obviously - but also volume and when and where. If you are an always-background-music person and they are an occasional appreciator of Benjamin Britten, that could make you sad.
4. Sharing the same definition of what constitutes a crisis and what is pant-wettingly funny. This might sound like another way of saying, check your beloved's sense of humour, but it's more than that. Their sense of humour-failure threshold needs to be on a par with yours. If one of you thinks that trying to catch pigs in a muddy field during a storm is hilarious, and the other one thinks it is serious and dangerous... uh-oh.
5. People-reading. If you have to explain why it was, in your opinion, the equivalent of nails raked down a blackboard when everyone assembled to hear Jill sing 'Summertime' a capella in the conservatory, that means your people-reading radar is out of sync. Which means... it's hopeless, is the truth.
6. Honesty rules. If you're the sort of person who asks the question "How do I look?", expecting the answer "Amazing!", make sure you pick One with a fully programmed honesty filter. If you would prefer the unfiltered response, even if it's "I can see your pants. And, to be honest, you're reminding me of Charlton Heston", then go for one of them.
7. Are you on the same page re the phrase "I love you to the moon and back"? Warm fuzzies or instant nausea? Sort this one out and you will accidentally cover a lot of bases.
8. Travel. Make sure you are either in Team Adventure, or team two weeks anywhere you can get a tan.
9. Agree on the thermostat setting. Not agreeing is practically divisive, and your temperature preference reveals a lot about your character. For example: low riders are more likely to enjoy outdoor activities, eat less, and spend less time at the hairdresser's.
10. Plate sharing. You've got to like each other's food.
That's the basic compatibility test. That, and your definition of "tightwad" and "enough to drink" and "just loud enough to not disturb the neighbours".