This is probably one of the best questions. We ALL doubt our abilites.
To differentiate between what is real phenomena and fake, it's really down to faith. They way I personally handle this, is I take into consideration what has been happening IRL, what's been on my mind, what I've been asking for from the Divine etc... If I was thinking of something scary before bed and I dream of that, it's nothing Divine, it's just your mind processing stuff. If you've been praying on something for a day or weeks or months and then you have a profound dream, upon which you find the interpretations to that dream directly correlates with that issue, it's Divine.
When you are having vision during meditation etc.. You have to practice with that really. Random thoughts/images will pop into your mind, and you have to decipher how that fits into the big picture.
For instance, when I meditate or pray I get images. Some of them are random and I allow them to "float away" so I can focus on what I'm doing, but sometimes the image is so completely innate to what I'm used too, you cannot ignore them.
Case in point, a few years ago I was doing a candle flame meditation and I saw a swamp and a woman lying dead in the water in a nightgown (like the typical white gown you see ghosts in) and she had long blonde hair. It immediately made me think of myself, but not in this life. It wasn't something in my subconscious, however it was very vivid. I have not done a flame meditation since then, as that was just too much "ew, weird" for me.
The best way for you to determine between your random thoughts and meditation "Divine guidance" is practice. Meditate each day and you'll eventually come to understand that which is being presented to you for guidance and that which is being brought to you as an expression of your daily thoughts.
Random thinking here, it's very late. Ask questions if you have any and I can elaborate