Hello.... I came across your warning about over-painted lustre ware on ebay more or less by accident --- very interesting. But, as you will know, the perpetrator is not unique. There will be less blatant and more skillful operators out there who also realise how easy it is to 'add value', especially by simply hand-tinting a transfer. It is pretty easy. I know because I was a professional restorer.
Although modern repair materials, such as epoxy resins and bake-on lacquers, give a very hard repaired/retouched surface, as far as I know they are still not as hard as fired ceramic glazes and enamels. True fired-on pottery enamels are often described as 'soft' because they are coloured glazes with a lower melting point, but they are still hard and glassy in nature. Paint, whatever kind, is not quite as hard as that.
The basic (non-damaging test) is simply to lightly run the finger tips over the suspect area: fresh paint has a slight 'drag', especially if thick (try it out on newly painted woodwork at home to get the feel) but this becomes less noticeable with age. If a special bake-on (meaning kitchen oven heat) lacquer has been used there will be far less 'drag'. The suspect area may well be slightly warmer to the touch than unpainted ceramic.
A needle point will scratch paint, but with care -- you can scratch/damage almost anything with a hard enough point and enough pressure. It is not generally recommended to use paint stripper, nor bleach, on gilding --- either might affect it (it is thin soft metal, usually fired at low temperature) and the lustre is a very thin soft coating, almost a gas-like layer. A tiny drop of cellulose (car paint) thinner can rapidly soften most paints.
In the case of lustre-ware, as you say, it is not easy to replicate the thin sheen but there are new products coming into the craft shops all the time, so be aware.