A good teacher

Is someone who does the thing they’re teaching. 

If someone is teaching you about business, have they started their own?

If you’re taking a writing class, does the person teaching the class write every day? 

When it comes to health, does the person giving you the information look after their health? 

Be wary of teachings or advice which come from people outside the thing they’re talking about.

You can tell how reliable the advice is from the person it’s coming from.