First you say something to your friends or anyone else, then you keep thinking about it the whole time. The most common sign of overthinking.
If you always end up feeling guilty for everything and keep saying sorry again and again, it's a major sign of being an overthinker.
When someone says anything about talking to you, you feel something bad is going to happen. You get anxious about what they are going to say.
You start thinking about past scenarios or your mistakes as soon as you hit the pillow. Not only does it disturb your sleep cycle, but you also end up feeling drained from the inside.
Another major sign is that you always think of the worst-case scenario of any situation. You would even imagine accidents or negative things which will cause you extreme stress.