There are many ways to check to see if it's safe to swim or not - checking the weather forecast may give you an indication but nothing is better than actually being on the spot looking at the state of the water as it can often be different to that predicted.
Even if you've travelled a distance to swim expecting the water to be OK if it looks too rough then leave it for another day.
There are a number of apps and forecasts you can look at ( I like XC weather ) - the important information is the wind factor rather than the sunshine factor.
Look at the WIND SPEED - if it's showing as anything above 20mph ( Beaufort Scale 5 / Fresh Breeze ) then you're likely to be facing some lively water. Below 10 mph ( Beaufort Scale 3 / Gentle Breeze ) is going to be pretty much calm.
Look at the WIND DIRECTION - if your swim spot is facing into the wind then that is likely to kick up the waves and make it rougher.
e.g. Tinside faces South so a Southerly wind ( coming from South to North ) will cause a rougher sea than if it is a Northerly as the land protects the beach from a North wind. Tinside also has shelter from the lido to the West and from the Corinthians from the East so that can mitigate the wind effect slightly in the close to shore bay area.
Cawsand faces East so an Easterly wind ( coming from East to West ) will cause a rougher sea than if it is a Westerly as the land protects the beach from a West wind. Rame Head protects it from the South so mitigates a Southerly wind.
The wind will also cause a significant wind chill if it is combined with a low air temperature which will make you colder much quicker.
The Met Office updates the forecast every 6 hours at midnight, 6AM, Midday and 6PM and most apps update in sync with that so if you're weather watching and hoping it will change then check in at about those times for updates.
Click on the links opposite for more detailed info.