How to see the solder mask layer in Altium PCB?
It's called &quot;T-Solder&quot; (top) and &quot;B-Solder&quot; (bottom) by default, and you can toggle visibility like any other layer. They're not mechanical layers, they're signal layer pairs, so when you flip a component to the opposite side the solder mask follows.
Here's the top solder mask for a TO-252 in single-layer mode:

And the same part with all layers displayed, but with T-Solder as selected layer.