Hello there, I’m fairly new here but have been lurking for long time. Weight, sizes and ring test are only preliminary tests. There are fakes around that could trick even a master. The only way is to check the metal itself. The first thing I did when starting was to seek a way to check my coins at home and not destroying them. And yes a x-ray spectrometry gun of way to expensive (unless you are a very serious stacker) there are devices that check the resistivity of the sample and will give you peace of mind, specially if buying from eBay.
A lot of the guy will run away from eBay like if we’re the plague. But I have got some pretty sweet deals there so won’t go into that.
A good resistivity analyser will be around 1200€ (...yeah I know) again there are some in eBay around 200€ but quite simple and will require some physics knowledge just to check a sample.