Winston put on the corner of the cover of his diary before he leaves speck of dust.
Step-by-step explanation:
When Winston Smith purchased a diary in a secretive way which he had bought to write his own thoughts in it which is also known as thought crime, he feared that if this diary is caught or discovered by the police, the police will read it and will monitor his thoughts.
So in order to see if someone has discovered it or not, he decides to put a small portion of dust on the cover of the diary on it's corner when he puts the diary on the side before leaving.