When to lead a spade?
In a nice spades strategy guide (written by Buckey) the situations to lead spades are the following:


Your partner has their bid or you can cover their bid. 
You know your partner bid high spades. Not leading spades might just give your
opponents a chance to set you by trumping with lower spades.
Either one or both of your opponents are void in any non-spade suit you could lead.
If you're trying to slough and have nothing better to slough in.
One way you can set your opponent is if you are two suited (spades being one of the
suits.) You can run spades and make your other suit good. (be very careful that you do
not set yourself by setting your partners bid in the process).
If your partner first led spades and is going for a set.
When trying to set a nil.