oxidizing vitamin c

I tell mostly by color because the smell can be misleading. On the other hand, some brands color their vitamin C serums orange, so you can't tell. 😔 Here is a picture of my vitamin c creme from a tube I didn't use for some time. The orange color means it's completely oxidised. As it goes to the right, it's less oxidised.

There are some tricks 1. Choose right version of vitamin C - ascorbic acid is a basic version, but very unstable. 3-O-Ethyl Ascorbic Acid is probably the most stable, but also most expensive vitamin C deriverate. MAP and SAP are used by many brands instead of Ascorbic Acid 2. Keep the product away from light and temperature - keep it in a fridge. 3. Use products with Vitamin C and Vit E + Ferulic Acid or Niacinamide. Ferulic Acid makes Vitamin C survive bit longer.

it smells weird and it's yellow-orange

The color of the product is yellow-orange. Then when applied to the skin, it triggers irritation

Depending on brands but if it starts out as “white” or clear and changes colour to light yellow, it has oxidised.