Yes, they are very clearly marked. As long as you're aware there is more than one choice at TTC and MK, you can't miss them.
At TTC, to change from MK monorail to Epcot monorail or vice-versa, you have to leave the platform and exit to ground level, then go up the ramp for the other monorail. But as was pointed out, there's no need to do that going from Poly to Epcot and back. It's easier and quicker to just walk Poly to TTC.
If you're arriving at Ohana after dark, the walk from TTC to Poly in the dark may be a little disconcerting for a first timer as the winding paths are lit only by "torches". But fear not! As long as you head west between the road and the lake, you can't go wrong!