My strong preference, and the things that has worked out best for me, is to actually be friends with a person before dating them. Since my partners are such significant parts of my life, it's pretty much inconceivable that they topic of them wouldn't come up.
In my online profiles on dating sites, I mention polyamory and my relationships quite clearly in my profile. That way anyone who does contact me will have no surprises. If the idea of me having existing relationships that I am not looking to replace isn't something they can deal with, then I'd rather know about that up-front than waste time and money meeting the person.
Turning the tables - if a woman that I had agreed to meet up with (I wouldn't call that a "date" - just a meet-up for coffee and a "getting to know you") told me only on subsequent meet-ups that they were in a committed relationship, then I wouldn't be too pleased. To me this sort of thing is part of the basic "getting to know you" process. All this, and a meeting with the corresponding significant others, I would want before we went out on what I would call a true "date".
I know others have different opinions on this, and this probably makes me quite old school, but this is my strong preference.
"Listen, or your tongue will make you deaf." - Native American Proverb