The stabilizing of products is a bit of an art.
I have just a tiny bit of background experience working with chemists and science of creating an emulsion that does not separate and especially does not separate over time when subjected to various ranges of temperature change.
Here's what I practice and share with others and that is to always shake a product before use. Just do it. Just make it a habit. If you're using a quality product that has no separation issues at all then it certainly won't hurt to shake the product before use.
If you're working with a quality product or even a lesser quality product and there some level of separation inside the bottle the shaking won't hurt and it will help.
Most bottled products use a solid color for the plastic so there's no easy way for the human eye to see if there's any level of separation. Some companies use a clear type of plastic so you can readily see if there's separation.
Outside of throwing products away because of the unknown... just make it a "Best Practice" to shake everything that's been standing on a shelf before use.
You can even go so far when using spray-on product to "clear the pipes" which means to shake the product and then for the first pump or two spray the product into a garbage can to get rid of the product that was in the nozzle or pick-up tube so when you do start using product from the bottle it is freshly shaken, not stale from sitting isolated.
It's just a good practice if you're going to be in this for the long run.
Also, if after shaking a product you notice the product does not appear to have it's normal look or consistency, then move to the next step of either shaking it more or writing it off.
