I think the advice to tune into your intuition is the best.
You can also think about this in terms of how this process is actually working. The subliminals are affecting the contents, the perceptions and messages, that are in your deep mind. If your mind is a tree, then what we’re working with here are the roots. When the roots of a tree are strong and healthy, you’ll see a healthy, vibrant tone to the bark, branches, and leaves. And best of all, you’ll see healthy fruit.
If our deep mind is the roots, then the bark, branches and leaves are like the thoughts, feelings, and perceptions in our conscious awarenesss. And the fruits? Those are our actions and our behaviors.
So, if you want to know if the sub is having its effect, you can directly check and observe. What kinds of thoughts, ideas, perceptions are you typically having in connection to the focus of the sub? And the best test of all: behavior. Take some action and see how it feels and how well it works. Does it feel solid? Does it feel right? If not, then feed the roots some more; either now, or you can come back to it in the future.
It can take a little while, I think, for the health of the roots to reach up to the fruits, so I’m not saying that you have to wait until everything’s perfect to move to the next stage. (Not that I’d even know that, anyway. I’m pretty new to this.) But I am saying that I don’t think it’s all just feeling around in the dark. I think there are some things we can observe to get clues.