Hi Orlando-
As an old hippie who inhaled everything in sight (50 years ago) I can tell you that drugs do not produce an experience identical to those in Monroe's or Bruce's books. Drugs provide experiences, that's all. We also get experiences from severe emotional states, high fever, lucid dreaming and so on. Of themselves these are just experiences, not of any specific value. The enlightenment that people get is due to the way that they handle the experiences, and the insights that they generate.
The late Dr Timothy Leary's version of the Bardo Thodol via LSD was essentially that the drug shuts down the ability of the mind to orient itself, so that a person must struggle with fear of death, imaginary dangers and threats, and must get reoriented in some other manner. That is similar to the passage through the stages of death in Tibetan Bardo Thodol tradition. The benefit of the experience rested specifically upon the practitioner who had to be in the proper frame of mind, have the right attitude and understanding, and would have to be willing to go through introspection in face of real or perceived personal flaws. Most of the other drug based approaches to mysticism have been very similar.
Prior to the modern marvels of chemistry the old yogis had equally effective methods. In place of taking a strange chemical, consider slowing your respiration rate down. (You can find this is Patanjali's Yoga Sutras and the Upanishads, among other places.) That will eventually stimulate your inner panic responses and produce the same state as a drug.
Another option (same sources) is bhastrika, "bellows breathing", through which hyperventilation is induced, and also leads to an exotic state. Again, it does the same as a drug.
Another approach is use of repetitive sounds and rhythms, either as a chanted mantram, or as a meditation seed. In this area, I'd suggest Michael Harner's Shamanic Drumming tapes. He also wrote a book that helps describe how they work. This is much the kind of thing Pulsar mentioned.
Dancing works, as attested by numerous African tribes. But you have to dance for one or two days continuously. Again, the experience occurs, but it's up to the actor to decide how to use it.
It is possible to simply hold your breath until you pass out, after which, if you are in good health and so on, your automatic functions will take over and breathing will resume. This gives an experience fully as intense as any drug. And, if you snuff yourself accidentally while doing yoga, you'll have done it for a good reason (meditation to know God) and will have a happy rebirth. Or so they say.
All of these require personal preparation, which is why there is a Noble Eightfold Path, or Patanjali's Ashtangha Yoga, or a Course in Miracles, or ... all the other teaching aids. If we simply spend our time in meditation on death, truth, love, and joy, we will reach the same general levels of enlightenment as we would have obtained through a series of (often horrific) drug experiences.
One thing that has stood out to me about Bruce's work is that he recommends helping others by soul retrievals. That doesn't sound like a drug - nor like a meditation - but it is at the core of discovering our own nature, as it reflexively liberates us just as fast as we liberate others. Aside from the karmic reactions involved, the same kind of preparation is needed and the same general kind of growth occurs, but it does so in a reasonable manner.
How to tell an hallucination apart from reality? I don't know - what's the difference?
dave