Being pro-life is a belief-based system. Any belief-based system determines it’s members by their beliefs, which is something you have to take their word for because you cannot read their mind.
When someone says they’re Christian (meaning they believe in Jesus Christ), you have to take their word for it that they really do believe in Jesus Christ because there’s no way anyone other than that individual can know. To be a christian, you just have to believe; though actions such as going to church or reading the Bible can help determine if someone is christian or not, you can only know if someone is Christian if they say they believe in Christ because you cannot know if they believe unless they say so.
Same thing with being pro-life. You have to simply believe embryos/fetuses are people; you just have to believe that, and the only way others can know that you’re pro-life is if you tell them you are. Actions such as going to protests and reblogging pro-life propaganda on the internet can go hand-in-hand with being pro-life (just as going to church can go hand-in-hand with being a Christian), but ultimately you cannot know if someone is pro-life or not unless they say so.
I didn’t think that was a concept I’d ever have to explain. It’s not NASA science.