This is where reading the river comes into play. You didn't mention any surface activity so I assume there is no hatch. You can always start with a dry fly if you see any adults flying around that may have hatched earlier in the day or the day before. Or, as I'm apt to do, try a generic attractor dry fly... such as one of the Royal series or perhaps an ant.
If you get no action, switch to subsurface. You can determine what manner of subsurface bugs are about by turning over rocks. Try to imitate the size and color of those bugs.
Then, you can use a subsurface nymph presentation of your choice... indicator, dead drift with a dropper, etc. If a hatch comes off you can always switch to an emerger pattern or a dry.
My rule of thumb is if I see no surface activity, I start with dead drifting nymphs after determining what manner of critter is living under the rocks. So I work from the bottom up.
If fish start rising, I change tactics.
Over the years I've learned that certain parts of the river look fishy and I know there must be trout holding there and I need to figure out what they'll eat.
So, I may spend a goodly amount of time in one section of the river before moving somewhere else.
WHAT!?!? There aren't 40 likes on GAT's post? I was reading through all this thinking, "when the hell is somebody gonna start saying to turn over rocks? For God's sake if nobody says I sure will!!!"
TURN OVER ROCKS.
Put a pickle jar lid in your vest pocket, flick the little buggers into it with a little water in it. Then you'll see their true color, the way they move, etc... and it is Taxon's preferred background to provide you with positive ID if you photograph em. Man, he's a wizard.