Its a little more complex than that. It's the value of current assets compared to all forecast future pension liabilities (though this is where actuarial expertise is also applied). Discount rate, life expectancy and inflation for example.
Although forecasts, they provide a snap-shot of whether assets today will be sufficient, need topping up, etc.
For public sector pensions, those assets don't necessarily exist (we don't have a ring-fenced pension fund like Germany for example).
The bigger the public sector, the larger the funds required to pay pensions in the future. If staff contributions don't reflect reality and public sector employers keep paying unaffordable employer contributions, the burden on the taxpayer grows.