Well, leaving jokes beside: stores that sell these products usually have a poster with pictures showing the different patterns defective wipers make on a wet windshield.
Compare these patterns with yours... and there you are!
Most manufacturers recommend to have them replaced twice a year. This may sound excessive, but vision improves a lot (in particular during night trips!)!
BTW: sometimes wipers are OK, but windshield could have get sand blasted, when driving close to the beach or in across a road in a desert.
Hope this has helped!
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