The only difference that is most evident between R188s and R142As is the font used on the external numberboards.
In the image above, 7219 is a converted R142A, while 7899 is a R188 C car built brand new.
The font used on the numberboard of 7219 (presumably Standard) is the same font used on all R142As (including the converted sets)
The font used on the numberboard of 7899 (presumably Helvetica) is the same font used in all R188s, and is similar to the font used on the majority of R160s.
Other than that one difference, the R188s externally look exactly like R142A.