I've been dilated 2cm and in very slow pre-labour for a month now. There is no sure fire sign that let's you know you are imminently going to go into labour, just have to wait for those contractions (which I know majorly *****, once you know the process is beginning it seems to take forever)
The baby could come at any moment but majority of women are dilated less than 3cm for weeks before they go into labor.