Instead people use a range of different rules of thumb to assess how growth is tracking relative to trend, or ‘potential growth’. Some of these are trickier to use in practice than others. For example, some estimates hinge on an estimate of feasible productivity growth.
But these will be subject to the risks that historical relationships no longer apply, or that productivity (which is not directly observable) is just plain mismeasured for a period. A measure of trend constructed in this way is okay to use as cross-check, but I wouldn't want to rely on that as my only guide.