Slitherine

How can you tell which units are your core units? Is there an icon or something?

Edit: Never mind - I RTFM.

In case any other new players are looking for this answer in the future:

In the unit list and around their strength number on the map, core units have a gold border around them and auxillary units have a silver border.

Another thing to understand up front is that "SE units" you get as a bonus do not count towards your core slots in a scenario. They are "free" slots/units to deploy. So if you deploy and see "0/22" units to deploy, it's worth checking you do not have undeployed SE units. They appear after the last regular/heer unit of the same type in your reserve list. Thus the SE infantry is after your last regular Infantry, ditto for armor. The armor is easier to visualize as it is in a Tiger camo. The infantry is, also, though harder to spot given unit size.