You should just be able to check your_table["key"] or alternately your_table.key. If it has no value (hasn't been set yet), it'll return nil.
Lua 5.1.2 Copyright (C) 1994-2007
Lua.org, PUC-Rio
> chars = {}
> chars.a = 5
> if chars.a then print("yes") end
yes
> if chars.b then print("yes") end
> = chars.b
nil