You didn't say what year your vehicle is? It was relatively easy on older vehicles. The firing order was stamped on the intake manifold, you'd get the number 1 cylinder to TDC on the firing stroke. With the distributer cap off you'd then see which wire was number 1 and go from there. The spark plug wired are also often times different lengths and if they've been on the vehicle for awhile will have certain bends that will aid in finding their proper location.
The best thing is to refer to the repairs manual in order to ensure that the wires are rewired in the correct order. Make sure to connect the plug wire between the number on the cylinder and the corresponding number on the distributor's terminal. You should find more details from the manual or by consulting with a mechanic. You may also find more information from the following site http://www.ehow.com/how_7523625_put-wires-correct-firing-order.html.