That's a very good question, one that has formed the basis of numerous critiques of Locke's philosophy over the centuries.

According to Locke's theory of perception, we can never form a complete picture of an object in our minds that matches the object as it actually exists out there in the real world, the world of time and space. That's because our minds deal directly with ideas, not with the real objects themselves. The best we can do is to have ideas in our minds of those objects, imperfect copies of reality that cannot correspond to the truth of the physical world.

So let's say, for example, that we pick a flower. We can touch it, smell it, even taste it, if we have a mind to do so. But although we can gain a pretty good idea of the reality of the flower based on our sensations, that's all it is: an idea, not the reality of the flower itself.

According to Locke, there are three basic elements involved in the act of perception. First of all, there's the perceiving subject (i.e., ourselves, human beings). Then there's the object existing out there in the real world (such as a flower, for example). Then there is the idea of the object that exists in our minds, which comes between us and the really existing object. That idea is the closest we can ever get to the reality of the empirical world.

That being the case, it is impossible on Locke's account to step outside our experience and compare it to reality. Locke simply assumes that there is a real world and that we can only gain an imperfect understanding of it through our ideas, our representations of objects.