Cleaning the VAT?

I'm changing resin colors (and brand, still abs like), and I'm wondering, how do I clean the VAT? I already removed the little puddle of resin and return it to the bottle as I read that I can use it later, I already wiped the FEP with a towel and water (it was water washable resin), how do I know that it does not have small bits of resin here and there? I know it is a noob question, but this is my first resin change.