Probably the best, rather expensive, advice regarding knowing you're really compatible with someone (before you even think of kids) comes from the Actor Bill Murray.

I think this happened when he introduced/gatecrashed a bachelor party in a hotel he was staying in.

Obviously one doesn't need to literally go that far - but such a journey, or a similar enormous challenge, will show you what might happen with the long-term challenges (and ultimately maybe more expensive ones) further down the line.