When an A-mount lens is attached, is there a function that allows the user to check the focus by magnifying an image?

Use the Focus Magnifier function.

Set it up as follows.

1. MENU - (Camera Settings) - Focus Magnifier
2. Press in the center of the control wheel to enlarge the image and select the position you want to enlarge using the up/down/ left /right side of the control wheel.
3. Confirm the focus.

TIP:
You have to set this setting each time you shoot an image. If you want to use this function frequently, it is recommended that you assign [Focus Magnifier] to a custom key.

For example, the Focus Magnifier function can be assigned to the Custom 2 (C2) button as follows.
MENU - (Custom Settings) - Custom Key Settings - Custom Button 2 - Focus Magnifier

You can also assign the function to a different key. For details, refer to the Instruction Manual.