I’d always done both imaginary stuff and astral travelling, with mixed results, and then when I studied core shamanism (where this question comes up a LOT) I found the difference was clear when working for other people, to heal them and to bring back info. That method gave me a structure and paradigm that permanently erased doubt once and for all.
It’s one thing to go into the astral and slay a horrible entity, another thing when you journey for someone who’s come to you with various problems, and you do the journey then you describe how that entity was ringing a bell all the time and coughing, and they say that sounds just like their horrible aunt who was an invalid who ruled the family from her sickbed, and made everyone live in fear.
I’ve anonymised that example a bit by changing details, but that’s the kind of symbolism that you get where there’s no doubt you’re seeing real stuff, and when the person reports that their problems got better or disappeared afterwards, then you know you’re really affecting the “real” world.
Healing isn’t always the go-to choice for black magicians but it’s a great way to build power and start learning what’s on the astral that’s just for you, and what’s going to make concrete changes in this reality.
It’s usually easier to mend people than destroy them using those methods, I think the general rule of the universe is for more life, not less, and even the most “everyday joe” kind of person will have guardians, ancestors and even godforms watching out for his welfare.
Even the “darkness channel” I discovered working with Ahriman was forms striving towards existance, and we can see this on the material level as well - leave a piece of bread out and mold will grow and flourish on it, cement over a yard and weeds will grow through the cracks, life’s kind of a cosmic law on its own in my experience.
In the case I based my example on, I healed the relative who was malignantly blighting people’s lives because that’s usually easier and more effective, and more beneficial to the surviving family, than any other option.
This answer is about how I did it - other people are bound to have different opinions and experiences.