This is a good question, but also incredibly difficult to answer. At least in ways that pertain to everyone or most. Many won't like this answer, but you will truly know it when it's there. You'll feel it. It'll be obvious or very close to it.
With my 2021's experience, he had 2 instances where we knew he was loved. He then had a few that liked him or liked him a lot. He's going the JUCO route and his offer was very generous. It very much matched all the "love" we were seeing and hearing. To describe it in one word, it was AUTHENTIC. Like most things in life, you know when it's the real thing.