How is sankhara stressful, and how does one see that?
If you define it as 'what has come to be or is coming to be', like the body, or feelings, then you might say that stress comes when they change against our will. But this is obvious, for who hasn't noticed this fact already? Feeling good while eating a decedent meal gives way to indigestion, orgasm gives way to neutrality. Happy gives way to sad. Easy to see, if you choose one series of events to pay attention to, eg contemplation of food while you're eating, the act of having sex, getting up in the morning, showers (who doesn't hate when the hot water runs out?
But if you consider the other side of sankhara, the volition, then the answer is more subtle, and does require mental stillness. For when the mind is much quieter than normal life, then each time the mind changes the object its focusing on you can see that this change of focus was volitional. This change of focus, when it happens in a state otherwise still, is jarring. Even very, very, very slight changes of focus are jarring, because they are based on a comparison of one state of being to another. 'not still enough!' so attention is directed more closely to the breath. 'not alert enough' 'where is the space' 'where is nibbana'. Each time there is a desire for something to 'be', then there is a change of focus, and what 'was' is now lost and often lamented, or what is gained is good, but just doesn't seem good enough, not for long.
In the end there is a realization that to compare the current state to a possible future state will result in shifting attention, which is bothersome. Comparing the current state to a past one is the same. Becoming attached to the current state and not hankering over the past or future state is better, but still not good enough. Even if you stop hankering over past things and future things, you realize that the current stillness and peace is also being willed, and with one perception of 'not quite', the mind will change places again, and what is lost will be longed for, what gained will not be satisfactory.
... Boy, that was long winded. Ha.
As you might gather, I do feel that the most subtle manifestations of sankhara and the dukkha arising therefrom are not easily knowable without jhana. So, I would suggest to anyone, anywhere, no matter what their life is like, that they give some thought to developing jhana. In regular mental states the changing of attention is so fast and all pervasive that it is not really noticed in and of itself - because there is no 'stillness' to contrast it to.