Having traveled public transport a lot (In the Netherlands, but also took trains/buses in France during holidays a lot), your best bet may be facial expressions and eye contact/looks.
What I've noticed is that people that really want a seat usually try and make eye contact with someone that's already in a seat, and they do so by roving glances throughout the bus/train carriage. They're not likely to focus on just one person but look at each person in turn. This shouldn't be confused with the cursory glance people use when trying to find a seat when boarding, instead, they usually continue their attempt to make eye contact even after the train/bus starts moving again.
They might look tired, desperate, or even a bit jealous (though that last one goes for a lot more people that have to stand on a bus/train). Chances are if you make eye contact with such a person and give them a small smile/nod/acknowledgement, they might feel confident enough to ask you if they can have your seat.
I always try to keep an eye out of signs of discomfort, such as shifting weight a lot, or clutching a handle, trouble with standing upright, etcetera. Usually people that do have trouble with standing on public transport will exhibit some discomfort.
In the end, though, much of transport etiquette still relies on people offering seats, instead of people asking for seats, either out loud or by non-verbal communication. There have only been a few incidents among 5/6 years of daily buses/trains that I can remember where people asked me out of the blue if they could have my seat, and where we didn't first make eye contact. There have been a lot more where I offered a seat after making eye contact and realizing this person looked like they could need a seat more than me.
So, even if there's some signs to look out for, in the end it's more likely you'll end up being the one offering up your seat. The signs I described can hopefully at least help you decide better when to offer.