Was this an OBE or a Dream?
As a general rule, if you doubt you had an OBE, you probably did not. But sometimes you can be almostly absolutely certain you did, yet still want to check on the facts.
The most common cause of doubt is thinking it might have just been a dream. How can you tell the difference? When you are out of body, the difference may be subtle, but it's there. You will feel a little more "real" than in a dream. You will experience a disconnected, floaty sensation. Your senses may be less ordinary than usual; things will look cloudy or hazy, sounds may be louder or less realistic, and you will notice that you can put your hands right through solid objects and actually feel what it's like to pass through them.
If you see objects that seemed out of place or surreal, this doesn't necessarily mean you are just dreaming the whole time. Sometimes when you have an OBE you will slip into a dream state toward the end. This is most often quite gradual. You will move from being out-of-body to starting to see dream imagery and will finally have fallen out of the OBE and into a dream. The clearest way to figure it out afterward is to think if you started seeing the dream images at the beginning of the dream or the end. If things became less realistic toward the end, it was probably an OBE followed directly by a short dream.
An obvious sign that it was an true OBE will be if you recall an exit or re-entry of the astral body from or back to the physical body. This generally will be a jolting sensation, and you will usually awake right after it.
The most reliable evidence of all is if you saw someting that you are able to verify. For example, perhaps while out of your body, you watched a friend or family member doing something or witnessed an event you would not have otherwise known was going on. By simply checking with the person you saw or looking into the event you believe you viewed, you can verify or disprove the experience beyond the shadow of a doubt.
Don't be discouraged even if you do conclude that your "OBE" was probably not an OBE at all. The good news is that once you do begin to have "dream OBEs", your mind is uninhibited and the door has been opened. Now the real thing is close behind and will catch up with you soon... though probably when you are least expecting of it!