This is a subtle & difficult problem.
Take for example gravity. Is it real? we can certainly see its effects: if I let go of a plate in my hand it falls to the ground. But I cannot see it.
There are three theories of gravity:
Newton: This was first theorised as a force. But we cannot see a force; we are forced to the situation above. But it was then discovered by Poisson that it could be seen as a field. Thus it pervades the whole of space. One can now certainly imagine, by analogy to light, spectacles which worn will let you see the gravitational field.
Einstein: Corrected Newton by showing that gravity was not a field but the curvature of spacetime. The nature of the thing that is observed has now changed. Can this be possible for something that is real. One point of view would be to say that to know something is real, is not to say we know all its nature; it may disclose further properties on probing.
Verlinde: Theorised that gravity is a thermodynamical property; that is it is emergent. Again the nature of Gravity has changed. This is rather like saying that a cloud emerges from all the raindrops that make it. What is real - the cloud or the raindrops? This brings us to the philosophical problem of the ship of Theseus: is the whole real, or its parts, or both?
Another angle on the real, which goes back to Aristotles substance theory; is to say what is real that is solely of its own nature and having no 'accidental' features; prime substance is that substance which is neccessary. The early Islamic & Christian theologians took this idea over to declare (whether one agrees with their theology or not) that the sole real substance, is the neccessary one of God. This is the position from which the rationalist philosopher Spinoza (that is a believer in Descartes onto-epistemological theories) took off from.
In Buddhism, this axiom of the Abrahamic religions is denied; there is no ground; there is no essence; everything is contigent - that is dependently arising. Thus nothing real.