You've always felt right at home with them, whether they're an old friend from high school or someone you've only known for a few months. You never have trouble coming up with things
Friends fight and grow distant, and it's natural for some friendships to end. But soulmates who are friends will always be friends. Even when they're not together, they'll think of each other. Soulmates who are just friends think about each other all the time, whether they know it or not.
When you spend time together, you're always thinking about each other. You can't really use logic to explain it, but your brain waves just match up. Maybe you're picking up on very subtle cues that no one else seems to notice, but either way, it seems like you've developed your own instinctual language.
A friendship with a platonic soulmate is a relationship that takes a lot of work and needs to be kept up. The difference is that it never seems like something you have to do. You'll want to put in that work, and because you like each other so much, keeping that relationship going will pay off in a big way. The actual maintenance will be almost painless.
No matter how close you are to someone or how well you get along with them, you are still different people, and there will be things about your friend that you don't like. This doesn't mean you aren't meant to be together.
You will be each other's cheerleader and shoulder to cry on without really thinking about it or setting out to do so. If you're having a bad day and your friend knows it and knows how to help you, that's a pretty good sign they could be your platonic soulmate.
You might not know it, but when you and your partner are apart, you both feel like something is missing. When you're away from your platonic soulmate for a few hours, days, or even a week or two, it feels like there's a hole in your life, and the only way to fill it is to be back with them.
There are some friendships and relationships where an uncomfortable silence can ruin the whole mood. But you could sit together in silence for hours with your platonic soulmate and never feel awkward.