The turn off the new trail used to be quite obvious, the old trail continued straight up the hill in an eroded ditch while the new trail went right. There were traces of blazes a few years back all the way to the summit.
I hiked it on Bloomsday last summer and the junction seemed relatively obvious. It's above the first "Slab Of Death*," pictured here:
and the Notch Trail makes noticeable turn to the right, and as peakbagger says, the old West Side Trail continue straight. The 26th edition of the
WMG briefly describes the junction. Some care was required to follow the West Side trail initially, but after five or ten minutes, the path became abundantly clear. Blazes, and IIRC, cairns still exist.
Have fun! It is, without a doubt, the best trail that I've hiked in the Northeast--though I have yet to do the Knife Edge. And, it is steep. Being able to do a layback is critical in a couple of spots, including the final pitch here:
Finally, be sure to: A) hike on a nice day; you want the rock to be dry and the views are as good as any; B) be sure to summit South Percy, too, as the views of North Percy are phenomenal.
*So named because, according to the
WMG, the mountain's lone fatality occured here.