As far as I am aware, the answer is no, there is no way of telling the state of the start battery (from within the car). Other third party equipment can be added to measure this.
If you are not using the car at least weekly, Lexus recommend running the car in park for at least one hour, to ensure the start battery is charged. During this time the hybrid battery will charge the start battery.
If you cannot trickle charge, then the car needs to be used as described above as minimum.