How to know positive and negative in breadboard

You decide which rails are positive and which are negative when you connect them to the + an - from your power supply or battery.

A breadboard isn't polarised per se. It's just a bunch of cross-connections inside a perforated plastic case. No active or passive components inside.

Some manufacturers may choose to paint indications regarding where to connect the positive and negative rails, but those are just a guideline. Ultimately it's your choice, you're free to connect + and - wherever you want in the breadboard rails.

As gre_gor said. A simple breadboard contains of rows and columns of connections but sometimes for convenience manufacturer marks or prints Positive and Negative symbol along the row as they are usually used to power a circuit