We know that the charge on the left is positive and the charge on the right is negative because

A the definition for the direction of an electric field line is toward positve charges.the definition for the direction of an electric field line is toward positve charges.
B the definition for the direction of an electric field line is the way a positive test charge would move.the definition for the direction of an electric field line is the way a positive test charge would move.
C the definition for the direction of an electric field line is toward the stronger charge.the definition for the direction of an electric field line is toward the stronger charge.
D the definition for the direction of an electric field line is the way a negative test charge would move.