Everything from ignore trace elements entirely, to test each one and dose it individually.
In fact, we don't really know what levels are needed/optimal/insufficient for many trace elements, and even testing does not tell all since, for many elements, the form dictates bioavailability as much as the concentration, and we have little to no data on the forms many trace elements take in reefs.
So it is a lot of trial and error.
My general suggestion for starting down the trace element road, is to experimentally try a mixed additive or set of additives that folks like, or dose a few one or two at a time to see any possible effects.
I personally only dosed iron and silicate, in addition to my 1% daily water changes and foods. But some things tested low and might have benefitted by dosing.