Bandai used a mask when painting the spines. so there's only paint along the outside edges. The spines on these figures are fully painted. That's not something that changes from overspray - it happens because the person/people making these new figures aren't using a mask to prevent the spines from being fully painted.
The Megaro-Goji isn't as obvious, but there's some things to look at:
1. The pupil (white dot on the eye) is much larger than most Bandai produced versions I've seen. On nearly every version of the the genuine Bandai it's just a tiny dot of white. On these figures it's about half the eye. However, Legion posted a closeup pic of his Bandai G73 and the eyes are pretty close (see below, Legion's Bandai is on the left) so now I'm going to have to say this isn't a reliable way to tell.
vs 
2. Lack of paint on the back of the head. I've got 2 of the Bandai figures (I realize this isn't the largest sampling) and on both of mine the spine on the back of Godzilla's head is painted, like the ones on the back of the body. This indicates that Bandai painted the spines after the figure was assembled. On the picture of the new figure show in the auctions it's clear there's no paint at all on the top spine, the one on the back of Godzilla's head. Looking at Legion's G73 (left) shows the same:
vs 
The seller is using the same picture for every auction so it could just happen to be a single figure that happened to not get paint on that spine, but if they're all like that, then it's a good indicator (unfortunately it's also something that's really easy for a bootlegger to correct).
3. High volume, no tags, HK seller - As previously mentioned the 73 was only released by Bandai as a tagged figure. The fact that a seller from Hong Kong (a well known source of bootleg figures) seems to have an unlimited supply, although all of them are untagged, well come on. Unless this is your first year in the hobby the only reason you wouldn't realize these aren't the genuine article is because you don't want to know.
My 2c,
Glow-nut
