If i wanted to get a mini 14 with a folding stock do i need a pre ban model?  How do i tell if it is a pre ban model?

Yes.

Serial number range.

No. If the Mini-14 in question has none of the "evil items" as defined by the AWB (federal law) then you may place one item on your Mini-14 no matter if it is a pre or a post ban.

Use a postban AR15 as an example.

The qualifiers for an "assault weapon" are:
Semi-automatic rifle.
Ability to accept a detachable hi-capacity mag.

Then you may have only ONE of the following:
Pistol grip
Flash hider.
Collapsable stock.
Folder stock.
Ability to mount a grenade launcher
Bayonet lug.

Your folding stock CAN NOT be a pistol grip folder stock, it must be a folder stock only.

In addition you must ensure that the overall length of the rifle is still within the federal law limit concerning minimum overall length (28 inches). If not you are crossing the line into the short barreled world of NFA weapons.

I don't know where you would look on the internet for a list of pre-postban Mini-14's.
You could call Ruger up, they should be able to tell you.