Objectively, how close are we (humans on earth) to overpopulated?
our ability to measure the degree of overpopulation is imprecise, but we can record and evaluate some measurements.
43% of the earth's land surface is altered by human use.
25% of the world's primary production is used my humans.
obviously, much of the earth's land mass is inhabitable and/or marginally usable for humans, and just as obviously, we already concentrate our use in the most advantageous regions. there's a lot of slop, but it is unambiguous that we use a much greater % than 43% of the land surface's potential to support humans.
primary production is an even sloppier figure. humans can use some production more intensively, some more efficiently, and significantly increase (or decrease) the total. nonetheless there are physical limits on primary production and we already monopolize a substantial proportion.
although a value judgement, and hence subjective, i assume that the notion that we do not wish to limit the economic prospects of the current and future human population to a third-world lifestyle rather than one more closely resembling our own is widespread enough that we can also consider as objective the notion that we must also include the expansion of resource use by the current population as part and parcel to growth in population size.
neither of the above metrics speak to real, but difficult to measure, concepts such as climate change, ecosystem services or loss of biodiversity (resilience), all of which impose other limits on the sustainable expansion of the human population.