Your smart watch may have ways of telling you how you're feeling today. Might I suggest you prioritize simply FEELING over any algorithm.
My athletes all report their daily wellness choosing one of five levels: fantastic, good, ok, subpar, and crappy. Waking up in the morning and thinking about how you feel today can take you a long way to an answer but I'll add some further help below. Once daily wellness data is available we can then keep an eye on the trends and adjust training accordingly. That may mean an extra easy day or even a day or three off if warranted.
What contributes to wellness?
Sleep Quality & Duration: The restorative depth of sleep and the ease of waking.
Nutritional Balance: Glycogen availability, hydration status, and digestive health.
Mental Stress: Cumulative load from work, family, finances, or academic commitments.
Training Load: The combination of volume and intensity across swimming, cycling, and running.
Mood State: General emotional outlook, including levels of vigor, tension, or anxiety.
Muscle Status: The presence of DOMS (delayed onset muscle soreness) or localized pain.
Health Status: The integrity of the immune system (e.g., signs of impending illness).
Morning: When you wake up in the morning ask yourself how you're feeling. Think about your general mood and how much you are looking forward to the day ahead. Consider if you are feeling resilient and ready for anything life (or triathlon training) throws at you. As you start to move how is your body feeling.
Workouts: Is the workout feeling like you expected or is it harder than anticipated? If your easy run pace is resulting in laboured breathing your body may be telling you that something's up. You could be carrying excess fatigue or even an illness. And think about how your body is moving. If it is missing that usual relaxed, smooth feel that too can be a signal that something is amiss. When you're not sure about whether you have the workout in you consider doing the warm up then deciding. Often we just need to get the heart and body moving to feel better. Conversely, if the warm up ends and you feel tired and your mood is still low, maybe it's wise stop there and to hit the couch for some extra rest instead.