There are a lot of signs. Many are very subtle, a completely RELAXED snake will tongue flick regularly. The body will have soft parts where the muscle is relaxed loose soft feeling. The breathing is regular, and movements will be smooth and free.
Inquisitive royals add lots of tongue flicks and alert exploration light tension and steady breath. Interested poking about and into things.
Stressed snakes there is often lots of body tension both in muscles in action and not in action. fast jerky movements few tongue flicks and erratic breathing.
Fearful royals are very tense, no tongue flicks. Rarely is there any movement static sometimes balled up or in strike position although I don't use that as an exact judge as it seems to be a 'default' position for them they seem to fall into it easily with or with out tension. Hissing is always a good fear or leave me alone indicator. Rapid irregular breathing too.
I use tension and tongue flicks as keys. No tension with tongue flicks is not a stressed royal generally but a stiff hard body with no tongue flicks is a great deal of stress.