I've tried everything: putting it out there before we even go on a date, telling after a couple dates, telling after a month or two (before the serious relationship starts) and being with someone for 6 months or more and never telling. Here's my analysis of each method.
1. Telling Right Away: The good thing is that you don't have to deal with the anxiety of rejection while going out with the person. The negative part is that there are a good number of people who won't be comfortable, so you are constantly opening yourself up to be rejected. Also if you live in a small community, you may not want everyone to know about your past, and this is a way for it to get around fast. However, all the people you go out with know about you and you won't be wasting your time as much.
2. Telling After One or Two Dates: This still allows for you to tell your truth pretty early before heavy feelings get involved. The problem is that the person has an idea of you already in their minds and they are now confused and have to re-evaluate. In my opinion, this method isn't great. It's too soon to allow a connection to form for them to be attached to you, and it's too late to be considered really up front and honest.
3. Telling Only When The Relationship Is About to Get Serious: The positive side of this method is that it allows the person to get to know you well before you tell them about your history. They likely have formed a good connection with you and have some attachments. It is likely going to be harder for them to reject you. However, they very well might feel deceived. They have started to form feelings for someone whom they now have a completely different understanding of. This can take a toll on the relationship, and cause them to distance themselves or leave. This also makes you more vulnerable because now your heart is invested, and could get broken if they decide to leave because of your history.
4. Not Telling At All: If you pass well, this may work for a little while (as long as you don't live in a town where everyone knows and is going to tell your partner). This really affects your ability to allow the relationship to develop to a serious level though. You won't be able to share much about your life without embellishing or re-writing your story for the sake of non-disclosure. You may feel closed-off and like you aren't being authentic. In my experience, these relationships never became serious because I could never invest my full authentic self into them. The relationships stayed pretty surface-level and casual even after a matter of months. Also, there's a chance they could find out and be REALLY pissed. This puts you at risk not only for your heart getting broken but for you being a victim of violence.
So with all of these options considered, I'd choose option 1 if you have the ability to be out in your community, and I'd choose option 3 if you are not comfortable being out in your community and/or it is not safe for you to be out as trans.
-Jo