Check the leaves or the nodes where leaves have fallen off. Maple always have leaves opposite on the stem. Liquidamber (sweetgum) have leaves alternate. Liquidamber also have a distinctive smell but you'd probably have to smell one first to recognize that.
Does not really matter what it is for now as both will respond in similar ways.
Your tree is just a skinny stick now so there are several possible avenues to follow:
1. prune the long stem which will stimulate new shoots from the stump. Let those grow and cut when they get longer to make a small, simple bonsai. This will usually result in a fairly skinny tree in a pot but some people are happy with that. More discerning growers refer to small, simple trees like these as 'mallsai' as they are grown in thousands and sold to unsuspecting punters online or just before Christmas in shopping malls and markets.
2. plant the tree in a large pot or in the ground and let it really grow for a few years. That will really thicken the trunk much quicker. When the trunk is thick enough cut down low and grow branches and a new top from all the shoots that will grow after the chop. That method should result in a thicker trunked bonsai with much more character and aged look.
The vast majority of good bonsai are cut down from larger stock rather than grown up from seed or seedlings.
Southern hemisphere spring means you have all summer ahead to do any cutting and growing. Spring, before the leaves come out is the best time for any root pruning and repotting. There is no real urgency about making decisions. As klong as you water and feed it properly the tree will keep growing and wait for you to make decisions.