I've been asked several times at meetings this winter if it would be possible to identify Palmer amaranth in conservation plantings at this time of year. My response has been 'I think so, but I've never tried'. To avoid having to use that answer in the future, I went to a field planted to pollinator habitat in 2016 that was known to be infested with Palmer amaranth. I hadn't been to the field previously, so I didn't know what to expect.
In the first ten minutes I failed to find anything that looked like a pigweed species, and started to question why I was out in a 'swamp' on this damp, dreary day. I finally found a pigweed, but the flowers would best be described as 'mush', and I had no idea if it was Palmer amaranth, waterhemp, or some other pigweed. I continued treading through the mud, puddles and ice until I eventually encountered several plants that I was confident were remnants of female Palmer amaranth plants. So yes, Palmer amaranth can be identified in January, but it will take some time to find recognizable plants.
It was clear that this field had been mowed in late summer, thus, the plants were only two feet tall. In fields where mowing wasn't done I think Palmer amaranth would be much easier to find (but that would probably depend on density of other vegetation). As with living plants, looking for the large bracts on the seedheads is the key for confident identification.