What are all of the other possible ways to check its purity? For example would you be able to differentiate between 95% purity SN and 99% purity SN simply by sight or smell, or weight?
Not really. Unless it was sent to an actual lab which would be extremely cost prohibitive and not to mention what questions they would ask on top of it, those are the ways it is test. Stick to the tests in the SN Bible.
(1) Weight: The purity grade affects its mass per unit volume but not its molecular weight. Molecular weight is an intrinsic property of a compound. For example, SN has a molecular weight of approximately 68.995 g/mol. If you have a 95% pure sample and a 99% pure sample, their molecular weights remain the same. If someone has 100 grams of a 95% pure sample, it contains 95 grams of SN. In contrast, 100 grams of a 99% pure sample contains 99 grams of SN. They weigh the same.
(2) Smell: There will not be a smell difference.
(3) Sight: For appearance, it might depend but not really useful for us. 95% might contain impurities, which
could lead to slight color variations and could have a less uniform texture, or the presence of other particulate matter whereas 99% is likely to be more homogeneous, often appearing finer without noticeable impurities. But would an average person who does not routinely work in SN really know the difference? No, probably not.
For more definitive analysis, other techniques like a spectroscopy would be necessary (not practical for us obviously).
And maybe most importantly above all: whether it is 95% or 99% makes no difference for SN protocol purposes, both are sufficient. Don't fret about this.
With whatever you seek, I hope you find everything you are looking for and get peace & serenity.