Visual Indicators
Most advanced players will know it’s time to restring purely through feel, but there are a couple of visual things to look for.
Notching
When you make contact with the ball, your strings will rub together and produce friction, which causes the strings to notch.
If you look closely, you’ll see grooves form where the main and cross strings intersect. This will be more evident towards the upper middle of your racquet, which is hopefully the area you make contact with the ball most!
If you see that these notches are close to breaking the string, or even just getting quite deep, it is likely an excellent time to restring.
Fraying
Natural gut and multifilament strings are composed of lots of tiny fibres that are intertwined. When they are freshly strung, they often have a coating on to protect them, but once this wears off, the fibres will start to fray.
This is a natural part of the wear for these types of strings, so some fraying is perfectly normal. Fraying can also be exacerbated by humidity and moisture.
However, beyond a certain point, the fraying will reduce the gauge of the string to the point it will break. So if you see fibres fraying at all angles and the string looks weakened, restringing is probably the right choice.