If you're heading northbound the spring is about a mile past Marshall Pass on route. It's a small pipe coming out of the hillside. There is usually water running across the road so it's hard to miss, even if you're heading southbound and haulin' down the dirt road.
If you've hit the singletrack you've gone too far.
RE: Monarch Crest Spring - Between Marshall Pass and Green Creek Shelter
I rode Monarch Crest [as a fall ride] for about 3-4 years, after knowing there was a spring along that section of the CT and missed it, even though it is relatively obvious when you finally find it.
The spring, with pipe out of the rock, is located on what I suspect most people would consider as singletrack, not doubletrack.
It's about a quarter mile south of the Green Creek Shelter. So, if you are north bound and come to the lean-to shelter, you need to backtrack.
During last year's CTR the flow-rate was pretty low, but I've never seen it totally dry. I've never filtered. If you can't trust this water source, what can your trust. Chock-full of minerals, and probably a wee bit of heavy metals too, but it's not your lifetime water source. Enjoy the awesome taste!
The spring is on the uphill [east] side of the trail.
South of the Green Creek Shelter meadow, it's on a section of trail in the trees, south [heading to Marshall Pass] of it is alpine [above treeline] single-track. Maybe there's another double-track spring I'm not aware of.
Look for a section of trail where you bike across a short bit of crumbled scree, which may or may not be wet.
The actual pipe sticking out of the rock is probably less than 10' from where your tires roll.
If you are NOBO and miss this, next water will be after you drop into Fooses. Finding water in Fooses is NOT a challenge.
Edit: Photo - Google Earth view looking east, "103" is Green Creek Shelter and spring is "105" or to the left [north] of 105. For sure the spring is not south of 105, where the trail comes out of the trees.
