Hmmm... by the definitions given, I am also a Dom as I am also unwilling to give titles unless I feel they are deserved and have a tendancy to speak out and act devils advocate when I feel it is needed... I can well understand the confusion. Trouble is with this internet thing is that tone and emotions rarely get transmitted accurately so it is easy to misread someone's intent and that can lead to misreading a personality. Generally, I think, making it simple and obvious helps.
Personally, unless someone has it written into their username (sub so and so, Master or Mistresss so and so) or has a tag line which gives you an indication (like many do have - telling us who their Master/Mistress or sub are) then I prefer to make no assumption about anyone until I have seen evidence - either asked them outright or sneaked a look at their profile. Yes, people, we are asked the 'what is your BDSM role' question in the profile for a reason... it is usually a good clue as to someone's status as sub/Dom etc.
Sorry, I shall switch off the sarcasm now...
I think this issue is up there with one of the main complaints of many lesbian Mistresses about being bothered by male subs despite it being clear on their profile that they are not looking for a male slave. Unfortunately, in my experience, profiles rarely seem to be actually read as much as they should be. I think if you are talking to someone in chat, for example, and want to know more about them then the profile is there to help you.