If an organic compound has dimethyl in its name than that means that it has 2 methyl groups. If they are on the same carbon they form a V in the bond line structure. An ethyl group on the other hand forms a 2 carbon unbranched zigzag in the bond line structure instead of the V that a dimethyl group forms in the bond line structure.
For the ethyl group it is standard on all bond line structures to represent it as a 2 carbon unbranched zigzag. For the dimethyl group it is standard to represent it as a V in all bond line structures.
As to how we know this it is because an ethyl group coming off of a carbon looks like propane if you just look at that carbon and the ethyl group. In fact some people in the early years of organic chemistry might have called propane ethylmethane before the prop prefix was used for alkanes. However if this was the case some of them would call propane dimethylmethane. This was because they were looking at different carbons as the central atom. If you look from one of the ends it looks like an ethyl group sticking off of methane. If you look at the center it looks like a dimethyl group sticking off of methane.