Okay so here we go!....
One of the ways to find out is to check the ingredients list, more specifically top/first 5 or so.
Ingredients are typically listed in order, from largest to smallest quantity based on weight or concentration least.
Most common are water, silicone , mineral or oil based products.
Sunscreen is also categorized as either physical or chemical .
-Water will be at the top of the list for most liquid products because water makes things blendable, more liquidy.
**** This does NOT mean that a product is water-based. Without water it would not be blendable/spreadable
-If the leading ingredients includes Aqua and other ingredients such as Ethylhexyl Palmitate or Glycerin, this is usually indicative of it being water based.
-If the ingredient list doesn't include water at all or aloe, good chance its oil-based.
-Cone/-methicone, and -siloxane words near the top of the ingredient list most likely mean a silicone-based product.
Clopentasiloxane, Cyclohexasiloxane, Cyclomethicone, Dimethicone and Phenyl Trimethicone are a few of the more common silicones used in skincare and cosmetics products.
-Generally any ingredients ending in "-cone" or "-xane" are silicones
-Silicone-based products tend to have a thicker, creamier consistency and feel more slippery or silky on the skin.
-Water-based sunscreens are usually lighter in texture, more easily absorbed, and have a lighter and thinner consistency. They tend to have a more liquid or gel-like texture and less often leave a visible residue on your skin.
-If Zinc Oxide and/or Titanium Dioxide are listed as the leading ingredient, this means it is a physical sunscreen.
Chemical sunscreens will contain ingredients like avobenzone, oxybenzone, octinoxate, tinosorb, etc,
-Oil-based sunscreens are generally chemical-based
-Water resistant sunscreens are usually made of oil, this is how it repels water and less will wash off when in contact with water. Although not always!
-Zinc Oxide also acts as a water repellent as it is not soluble in water. Zinc Oxide is mineral based and oil-free .
-Another way to understand the formula is to apply some of the product onto your skin then try to rinse it off with just water. If it washes away easily, it is most likely water-based.
I Hope this helps answer your question!