The original kit has 3 perishable parts.
1. First developer
2. Color Developer
3. Reversal bath.
They are packed under nitrogen, so once you open them to the air, their 1 - 3 year shelf life goes away. That unopened shelf life depends on date of manufacture and how it was kept getting to you (hot, cold etc).
A one part developer is not as stable as a 2 part developer. Therefore the First Developer is less stable than the Color Developer which is usually 3 parts. The First Developer should be light brown to clear.
The Color Developer, if in 3 parts, only goes bad if the small bottle (usually B) turns brown or black. If amber to clear it is fine.
The reversal bath gives no indication that it is bad, other than giving bad results.
All liquid kits can be mixed up in small quantities. Powder kits cannot. After opening, liquid kits usually last as concentrates for about 6 months or less. The mixed working solutions will last about 6 weeks in full bottles.