Check the live streaming of Nazaré Praia do Norte and main Village beaches.
It is the most practical way to see the beach, by desire or curiosity, on days of great swell or on calmer days. It helps to decide when it's worth going live the sea.
Here you can see the surf forecast report of Praia do Norte in Nazaré.
Usually when the swell is above 2 meters in the forecast (actually the Nazaré canyon amplifies this value), the period of the waves is above 13 seconds and the wind is not very strong and offshore, we have good conditions for big waves!
Check the windguru forecast table bellow: