If any, you will find them in the deeper pools (impoundments) where there also cold springs - usually below tall bluffs.
As far as targeting holdover rainbows - well, it would be blind luck to find one.
Here was a surprise July rainbow in the Guadalupe N. fork a about a quarter-mile above the confluence.
This fish probably migrated down from Bear Creek winter stocking by Hill Country FFF, which was about a 10-mi move.
This was another surprise - my buddy Alex landed a 22-inch rainbow at Mueller Falls - this fish most likely migrated down from Joshua Creek, which is quite a haul.
A friend who lived on Lake McQueeny frequently caught rainbows from his boat dock - they moved down from state and GRTU stockings in the tailwater and Landa Park.
The state record brown came from Lake Dunlap - same thing.
I can think of several holdover spring pools in the Guadalupe tailwater below mile 10
(and, of course, many more above mile 10).
"Mystery Pool" - you could hide a pickup truck between those submerged rocks
This is a place to target holdover rainbows (maybe the best place)
October holdover - one of many this day.