No doubt 830 million years is an estimate with plus or minus some number of years possible.
As for how, there are dozens of radiometric & other methods, absolute & relative, which could be used, depending on the exact conditions at the site in Australia.
Good chance that a number of different methods were used to confirm and narrow the estimated range.
The same experiments have long been done on other salt crystals, younger, from other deposits.
The difference is these particular crystals are found to be older than those others, the oldest such found so far.
Or they eyeballed it. Has about the same amount of accuracy.