If we leave out jokes such as "put a fingerprint on it" or let it have "ON" light, the possiblities are quite limited. Without an unpressed reference button only the placement under the surrounding surface is a strong enough glue. The right illusion is difficult to achieve, very easily it seems to be a disk hanging in the ceiling. See NOTE 1.
This is actually a screenshot from Illustrator, but it hasn't anything which isn't repeatable in Photoshop.
The button is vertically squeezed to elliptic to make an illusion it's partly behind the surface. That illusion is tried to make stronger in following ways:
- The surface plate is partially visible in the upper side of the hole (a grey disk)
- the button has gradient color, darker in the upper side, opposite than the surface gradient
- a little downwards shifted darker version of the arrow just below the white arrow; this is maybe useless, but it presents extrusion, the arrow also has some thickness.
- the arrow is shifted a little downwards, it cannot be in the middle, if a part of the button is hidden
I tried also to have a solid blue button with a pinpoint shadow at the upper edge. Here a blurred grey sickle has been inserted. It unfortunately somehow makes the wrong "hanging disk" illusion stronger. I tried to compensate it to make the apparent thickness of the arrow bigger.
Just to test it in Photoshop I made this:
Bevel&Emboss layer style is used to make
- the thickness of the arrow and
- glossy edge for the hole in the frontplane
This can seem complex. The PSD is available for further examination here https://www.dropbox.com/s/scalmtcxsojuvku/ButtonPressed.psd?dl=0 The link is valid in February 2019.
There's other cases where the wrong illusion also has been a problem:
Adobe Illustrator: Deboss or Engrave Effect
Adobe Illustrator: Making a shape appear to be "carved" into a surface