- 1st Path, complete one cycle of the Progress of Insight
- 2nd Path, complete another cycle of the Progress of Insight
- 3rd Path, "perceiving the emptiness, selflessness, impermanence, luminosity etc. of sensations in daily life"
- 4th Path, "untangled the knot of perception, dissolved the sense of the center point actually being the center point, no longer fundamentally make a separate Self out of the patterns of sensations as they used to, even though those same patterns of sensations continue."
The first 2 paths are fairly straightforward, based on completing 2 Progress of Insight cycles. Completing a cycle is marked by the "experience" of cessations or fruitions. The Progress of Insight cycle was recognized by the time of the Vishuddhimagga, a 5th century meditation manual, and was slightly updated by Mahasi Sayadaw in the 20th century. The cycle is described in the MCTB and in Mahasi's works. If you really want to geek out about it, Daniel has an interesting detailed view of the cycle with jhanas, nanas, and sub-nanas in a kind of spreadsheet form. Keeping a daily meditation diary can be a big help in determining where you are in the cycle, but eventually the cycle becomes less important.
Kenneth Folk has proposed a 9 stage model that is very similar to Daniel's Revised Four Path Model, but adds a stage at the beginning, basically acknowledging the importance of the Arising and Passing, a point on the Progress of Insight cycle that occurs early in 1st path which can be a very big experience. Psychedelic drugs sometimes take one into this territory. Kenneth's model is then virtually identical to Daniel's for the 4 path stages, and then adds a number of stages to flesh out the territory after 4th path. A good personal description of Kenneth Folk's model can be found at QuietMind Meditation Co.