As I keep and breed both species for about 5 years now, I can say that they first of all differentiate in adult size, further in colouration and width of bands and nuchal loop, iris colour and length of tails (tails of G. lichtenfelderi are shorter and do not end very thin). The most important fact on my animals is locality data as I have received my G. lichtenfelderi from a very well known russian Goniurosaurus breeder who got them from some russian herpetologists, who described them as G. murphy in the past. Even though G. lichtenfelderi has never been imported in big numbers in comparison to G. hainanensis (shipments are still declared falsely as lichtenfelderi). All those facts make it very easy for me to distinguish between both species. The bigger problem is to tell the high and lowland forms of G. hainanensis apart. I would say that this is nearly impossible without exact locality data, as they vary very much in the wild.
Hopefully these two pics of currently hatched G. lichtenfelderi and G. hainanensis will show you the differences on my specimen on the first look.
G. lichtenfelderi (2 weeks old)
G. hainanensis (3 weeks old)
Cheers
Thomas