All experience is subjective. This means that it is impossible to know for sure how a person experiences either the real 'walk in the snow', say, or an imagined one. However, fMRI scans have shown that there is a strong correlation between imagined experiences and real ones. A more experienced person than I could possibly explain the areas of the brain involved.

There are many practical ways of causing such a 'mental journey', the most obvious being hypnosis. Whether you believe hypnosis to be a seperate state of consciousness or not (my money is on it not being seperate, and research is starting to support this position), it can certainly evoke very vivid memories and a highly detailed personal subjective experience.

Other ways include guided visualisation, lucid dreaming and meditation.

For me, the most vivid internal experiences have been achieved through lucid dreaming.

As a follow-up to the above. I still have a crystal-clear memory of my arrival at graduate school: driving down the road, pulling into the parking lot etc. The problem is the memories are at odds with the reality. The image of the building in my mind does not match the way the building actually looked, as well as several other anomalies. So, how can we ever be sure that we are truly ‘reliving’ an experience without an objective check on the accuracy of the experience?