phase_dancer
Bluelight Crew
How can you tell how fresh the reagent is?
Make it yourself and put a date on it

In short, if buying the reagent and it's not dated, you can't. You can go by appearance though, which is a pretty good guide. When the reagent begins to age, polymerisation of the formaldehyde occurs (along with other reactions) which produces a brownish colour that darkens with further aging.

