Advice on best methods to test silver

[Jazzman111]

I have been collecting American silver folding fruit knives for the past year of two. As anyone else following that hobby knows, it can be fairly frustrating because American silversmiths, unlike their British counterparts, were under no obligation to mark their knives to indicate whether they were made of coin, sterling or silver plate. I am considering acquiring whatever appropriate materials used to test my knives' composition, but I'm seeing on this and other websites information that can be a bit confusing. Would greatly appreciate some experts' advice on some questions:

1) There are acid test kits that are fairly inexpensive. But how reliable are their results? Can they distinguish, say, between .900 and .925 fine? Can they tell the difference between the latter and a silver plated surface?

2) I read about XRF spectrometers, but they look pretty expensive. Are there other methods that fall between the latter and acid testing?

3) Is there some reliable way to determine whether a knife is silver plated, other than inspecting it closely for tell-tale wear of the silver plating and exposure of the non-silver base metal?

Thanks in advance for any help you can provide!