Overtime fences can be prone to some general wear a tear especially if exposed to insects, water and other environmental factors. Some easy to spot signs that show your fence may need repair are splintering, warping and discolouration.
Problems such as small holes can be easily fixed with wood filler however damage such as warping are better resolved with a new panel being fitted.
If you believe any of these repair options will be too costly you can always seek to replace your fence. A good rule of thumb is that if more than 20% of your fence panels needs replacing it may be time to replace the whole fence.