1. You're hyper aware of subtleties in their environment.
HSPs usually notice things that other people miss. Maybe it's a new haircut or furniture that's been rearranged. Sounds can startle them easily, and they're more sensitive to certain smells and fabrics.
2. You're easily affected by others' moods.
Everyone is affected by the moods of others, both positive and negative, but the effect is heightened in highly sensitive people. It's great if you're around positive people, but not so great when you're around people with heavy energy.