The activity of the vagus nerve is difficult to measure directly, especially given how complex it is. But because some vagus nerve fibers connect with the heart, experts can indirectly measure cardiac vagal tone — or the way in which your nervous system regulates your heart — by looking at your heart rate variability (HRV), which is the fluctuations in the amount of time between your heartbeats...An abnormal vagal tone — one in which there is very little HRV — has been associated with conditions like diabetes, heart failure and hypertension...A high HRV may signify an ideal vagal tone. The typical range of HRV is between 20 and 200 msec.
I will give my own experience...I have been using an Oura Ring since December 2021, and more recently an Apple watch, to monitor nighttime resting heart rate, HRV, body temperature, and respiratory rate. By now I have documented numerous instances of a correlation - occurring over a period of several months - between subjective well being, average nighttime HRV, and duration of deep (restorative) sleep. (See the plot below showing HRV and duration of deep sleep over the past several months). During periods of stress my average nighttime HRV decreases to ~20 msec and remains relatively constant throughout sleep, during periods when I am feeling open, chilled out, and flexible average nighttime HRV has increased to ~100 msec with large variations during the night. I've also played with techniques meant to tweak parasympathetic/sympathetic balance and found that delivering mild shocks to the body by perturbing breathing or using biofeedback to enhance HRV can correlate with increased average nighttime HRV and daytime sense of well being. Even though I take myself to be an unbiased observer and don't think that I am just feeling what I would like to feel - less stressed and more chilled out - it is important note the usual caveat that any human reports might be biased by a placebo effect.
Screen shot from the Oura Ring web interface: