if by youngish you mean 3"-4", then i'm not sure i can tell you clearly what the differences are, since congicus are still wild caught, and they don't come in that small.
but if you mean around 7"+, then the differences are very clear. head shape is different. congicus has a narrower body. markings are different (and it would be the same differences you see on larger individuals).
P. endlicheri congicus has neater, narrower, typically fainter, and higher number of vertical stripes (typically 7-9).
P. endlicheri endlicheri has wider, more irregular (and sometimes zigzaggy), bolder, and a lower number of vertical stripes. caveat: some of these characteristics, e.g. wider stripes, refer to wild caught individuals. There are some farm bred ones with narrow stripes, horizontal stripes or other weird patterns.