Does it have any phono cards fitted?/How many
What are the filter caps like on the PSU? These are usually scorched by the power regs and you can see them visually..
The age of the mixer is printed on the output transformers
Has the mixer been modified?
Any loose connections inside, leads come loose due to cold solder joints etc?
How do the pots feel? Are they smooth? Do they sound ok?
They are old beasts so try to check as much as you can before putting all your money down, if it's got any small faults at least you can start bargaining for a better price.
--The future scares me, any chance of a lift back to the past any one?--