Good communication is essential to avoid problems. Monitor your partner very carefully; check to see whether there is any unnecessary discomfort; whether or not a rope could be adjusted for better effect; a knot digging in or a knot chaffing can adversely affect the whole experience, just as a stone in your shoe can become the focus of your attention.
If, as the one being tied, something doesn't feel right, or you're not happy about the situation, never be afraid to say so. Your body knows best so listen to it, and don't try to be a hero or exceed your limits - when you start doing this, you start getting hurt.
If as the rigger, you are in any doubt whether your partner is ok, ask. Non-verbal signals will also tell you volumes, and part of the skill is interpreting these. Watch their face; observe breathing and skin temperature. Some people prefer the 'silent double-squeeze' technique, i.e. the rigger gives a squeeze of the hand to their partner, and if everything's okay they return a couple of squeezes. This is good because it's proactive - as sometimes the person being tied might be on cloud nine and not thinking about their own safety.
I personally prefer straight communication to safe words, it's less confusing; though playing games where protest might be part of the role-play, it's sensible to have unambiguous safe words, for example "red” equals "stop immediately”. However I feel that if you have to use a safe word it usually indicates a lack of careful observation by the dominant partner.