Read Write CMOS
I've recorded the old time, set the time to 1:59:58am on the last Sunday in April - the chip doesn't check the day matches the date, so the actual date doesn't matter as long as there isn't another Sunday after te date selected, so using 30th is guaranteed to work.
Then wait 2 seconds and read the time again - and it should be 3 am if the BST function is working, and then restore the old time plus the amount of time elapsed in the meantime, which I have been counting with TIME.
In fact, making sure the clock didn't lose any time during this function was the most difficult part of this util! This is because it isn't possible to read or set fractions of seconds, and this operation doesn't take a round number of seconds to do. So, to get round this, I kept reading the seconds from the clock until it changed, then I waited until a round number of seconds had elapsed after checking the BST function, so that I could be as sure as possible that as little time as possible was being lost.