My Fluorescent sign just went. I've 2 signs tied in together some how with a total of 4 ballast. One of the signs has been out for awhile and I've been running only one of them. The 1st thing I did was replace 2 tubes and that didn't work. There is power coming in. What could it be ?

A bad ballast. An electrician will have no problem fixing this for your business.

Is there a quick way to know which ballast is bad ?

To be honest, no. it is a matter of tracing wires, or disconnecting ballasts and turning power back on to see which ballast is which.

Where is this sign located?

It's above my store front. About 8' above ground

Stan is correct, other than tracing wires from the lamps not working to the ballast that operates those lamps, there is no trick to determining which ballast is defective.

Many times a bad ballast is extremely hot to the touch, much hotter that properly operating ballasts. Sometimes it is dead cold.