There is no reason that chemicals are the only actively defining part of our mind. It may very well be that chemicals are everything we need to messure, it may also turn out that we need to messure a lot of other things. Point is we do not know, and have a growing problem to figure it out.
Structure. An integrated circuit is just doped silicon, but understanding its chemical composition tells you practically nothing about how it actually works.
This "chemical composition" is like reading packets off a broadcast network. It's not the full state, but it's good enough to reverse-engineer what's going on at a high level.
Blood work, including blood neurotransmitters and hormones, can tell you a lot about someone's state of mind specifically because of this.
Maybe if you only consider the chemicals that are normally considered.