Hi CM- I am an easterner and have no experience with these species. All I would be able to do is go to the Sibley guide. He says the two species are not reliably distinguished visually, just by voice. The Sibley e-guide mentions that Tropicals are often seen in pairs or small groups. However, Couch's appears to be a bit more widespread and commoner than Tropical in Texas. Perhaps we have an expert on this species group lurking somewhere in BPN?
Couch's and Tropical kingbirds can often be distinguished by wing morphology in that the 10th primary (the outermost one) is longer than the 5th primary in Couch's but shorter in Tropical. Having the bird in the hand really helps but sometimes good images show these characters.Also, and a bit more subjective, Tropical Kingbird has a more deeply notched tail than Couch's, especially in the eastern populations. Not ideal but the image with the wings spread seems to show p10 > p5 and the bird in the first image does not have a particularly notched tail (the birds in the middle image are molting so it is hard to judge). Both features would lean towards Couch's Kingbird.