A base heater usually has a pair of openings or doorways in the back wall of the firebox, just above the firepot. There will be a door that is controled by an outside lever that closes off one doorway, this makes all the fire's hot exhaust gasses have to travel through the other doorway.
On the back of the stove will be a box-like chamber that the two doorways open into.. the doorway with the door, when the door is OPEN will exhaust directly to the exhaust port on the back of the stove. This means the hot exhaust gasses bypass the rear box, and go directly to the flue/chimney.
The other doorway, which does NOT have a door to close it off, opens to a passageway that goes down the back of the stove, through the base of the stove, under the ashpan compartment, then back up the other passageway in the rear box, then out the exhaust to the flue/chimney.
So the easiest way to see if a stove is a baseburner is to see if there is a box on the back of the stove, with an exhaust on the back of the box.
Then check to see if under the floor of the ashpan compartment there is another compartment, usually a few inched deep, and usually there is a small
cleanout door somewhere in this base compartment..
A quick note: if the stove has the narrow, tapered 'waist' and you can see the tapered round 'funnel' shape of the firebox, and the exhaust comes out of the top of the stove,, it is NOT a baseburner..