There are usually three main sections in the workings of a full-range speaker enclosure; the crossover board, the low-frequency driver and the high-frequency driver. The most common parts to fail are one or both of the drivers. A simple test for a speaker cab (or an actual speaker) needs nothing more than a 9 volt battery and a reliable speaker cable with bare wires at one end and the connector for your speaker enclosure at the other. If you plug the cable into your speaker enclosure and touch the two bare wires to the terminals of the battery (try to get positive and negative the right way around), the enclosure should make a 'clicking' sound. If it doesn't then you either have a fault with the internal crossover board, or both low and high-frequency drivers have blown. If the cab does make a sound, then you can put your ear to each driver in turn and use the battery test to check whether they're working; if either driver makes no sound, then it's probably blown. You should confirm this by removing it from the enclosure and touching the battery to the terminals of the driver. Again, if the driver doesn't click, then it's blown. This is an effective but crude but way of fault-finding and it's worth bearing in mind that just because a driver is working, it doesn't mean that it's working faultlessly. Crossover board repairs will usually need to be carried out by a service engineer; high-frequency drivers can usually be repaired by replacing the working part which is called a diaphragm, and low-frequency drivers will usually need replacing, re-coning or in the case of manufacturers like Peavey you can buy a 'field-reaplceable' basket assembly which is quite easy to fit.