Maybe try one if Adafruit's "example" sketches. If you do not have a spare I2C device, you really are at at the mercy of luck trying to find intermittent issues.
If using jumper wires, replace them one-by-one. If soldered, disconnect power and use a decent VOM/DVM to measure the resistances.
Not everyone can afford the luxury of 2-of-each part, but with substitution, it is the best method.