You might be able to use either the "Destination In" or "Source In" blend mode.
You would need to make the magnifying glass out of two sprites,
(1) the image of the magnifying glass, and
(2) a "mask" for the glass area of the magnifying glass.
This mask sprite would have it's blend mode set to "Destination In" if you wanted items under it to become visible as the glass mask passes over them.
Alternatively, you can use "Source In" if you want the glass mask to reveal objects above the glass.
To see examples of these blend modes in action, you can open the blend modes example project.
In Construct2, from the main menu, choose New, and scroll down to, and open, "Example: Blend modes".
(You can also search for "blend" in the search field, instead of scrolling to find it.)