Start at the Guaje Canyon Trail #282 at the Pajarito Ski Complex.
Hike north along the trail, which is an old road at first. About a quarter-mile in, bear left at a junction. Follow the trail through the trees, gaining slightly and level in parts, for about a mile. Leave the forest and cross Canada Bonita (very beautiful). The trail re-enters forest cover, crosses a locked gate (hikers are welcome), opens up again and comes to Pipeline Road. Bear left at a Y-junction and continue on the trail another mile-plus to a point where it turns sharply east on the SE flanks of Cerro Rubio Peak. The trail loses about 170 feet of elevation and comes out to a narrow promontory above Guaje Canyon. For the first time Caballo Mountain is visible across the canyon. Enjoy the views.
Find a steep set of switchbacks on the promontory's north flank and descend into Guaje Canyon, eventually entering into the verdant canyon bottom amid huge fir and aspen stands. About a mile later the trail junctions with the Caballo Trail (#277). Turn left onto this trail. Follow #277 steeply up the southwest face of Caballo Mountain. Some deadfall will require you to make short bypasses, but the route is always obvious and easy to follow. Up high the gradient lessens and heads due east. Finally, the trail breaks into the open again in a beautiful meadow just south of the summit. The tread peters out amid the meadow's grasses despite being shown on the map (I surmise just about everyone who hikes to Caballo comes up this way and not from the east).
To get to the summit, bear northeast into the trees and work your way up to the very broad, gently rounded top. There are no views.
To get to the Los Alamos County Highpoint, walk about 200 paces southwest of the summit, still in the trees, and locate a downed barbed-wire fence and orange National Forest markers. This is the Sandoval/Los Alamos County line (the summit is in Sandoval County). Walk the fence line to its highest points, and call it good. We built a small cairn beside a tree at the likeliest highpoint location. Again no views.
The best views are out in the meadow no more than 100 feet south. Pajarito Mountain is to the south, Los Alamos down below, and Santa Fe Baldy across the way to the east. Return the same way.
Comments on the trail: the trail has many ups and downs. Roughly speaking, the trailhead is at 9,220 feet, tops out at 9,651 north of Canada Bonita, descends to 9,480 near the Pipeline Road, ascends to 9,615 shortly thereafter, drops another 100 feet, gets up to 9,617 near Cerro Rubio, descends to 8,621 at the 277/282 trail junctions in Guaje Canyon, and ascends to 10,496 at the summit. The one-way net gain is about 1,260 feet but close to 3,400 feet when the bigs drops and re-gains are figured into the trip. Plan on 7+ hours for the 14-mile r/t hike. Views are often quite spectacular!
Usual hiking duds, pants, water (Guaje Cyn may have streams, but bring your own just in case).