Heres one way i heard of.
In your lucid dream yell out "Show yourself dream guide!" and turn around. Chances are someone will be behind you, but that doesn't necessarily mean that entity is your dream guide. To find out, ask for it's name and ask if he/she is your dream guide. If the entity says something random like "I like peanut butter and penguins!" than chances are it is not your dream guide. Also, keep in mind that dream guides tend to be more detailed in appearance and intelligent. Some also say that if you are unsuccessful finding your dream guide, it can mean that your dream guide thinks you are not ready. If this is the case, work on improving your frequency of lucid dreams and your dream controll. Good luck!