My favorite symptom is a controlled sense of humor. Both people who turn everything into a joke and people who never laugh are seriously low on EI. This includes comedians in their non-performance parts of life.

Another great symptom is the ability to wait. At kindergarten levels, this is the 'count to 10' trick. At more advanced levels, this is a part of great timing instincts: acting at the right time rather than the most tempting time.

A third symptom is knowing when you can argue and when argument has no hope of changing anything. Low EI people don't get that reason doesn't actually work until all parties have explicitly agreed to base a discussion on reason. Between reason and any kind of unreasonableness, there is no contest. Reason will always lose. Even if you win the argument, you won't win the situation. If somebody argues that there are no black swans and you produce one, you still won't "win" because the other person may resent you for proving them wrong. Resentment is almost never a useful outcome to shoot for, which brings me to...

The fourth symptom: avoiding 'winning for the sake of winning' situations. Strong EI people are never motivated by "told you so" reasons or worse, revenge.

There's a lot more...but those are some of the basic ones.