Click the Apple menu item (top left of the desktop), and then About This Mac. If the dialog box that appears shows a Memory tab then your Mac’s memory CAN be upgraded (that is, it uses SO-DIMM modules).
If your Mac has memory soldered to the mainboard then you won’t see a Memory tab in this dialog box, and this is because the memory CANNOT be upgraded. Not even by Apple technicians.
See below for just such an example, taken from a mid-2015 MacBook Pro. This Mac has memory soldered to the mainboard, so there’s no Memory tab. This is the trend in modern Macs outside of the Pro and larger iMac range, alas.