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.