How can I tell that the boost is recharged? I have not noticed any clear indications on when the boost is ready.
There is a coloured circle around your ship. The top half is blue which represents your boost, the bottom half is green which I believe is your overdrive meter.
The blue part will be almost faded out when your boost is full but when you activate it you'll see it light up and deplete. After you stop boosting it will refill and then fade out again when it has fully recharged.
It's must be fully charged to be used so short bursts are advisable to minimise the wait before you can use it again.