I had a good colleague a few years ago, who started at my company at the same time as me. The last time we worked together was around five years ago and we didn't have any contact since, but we still had each other's phone numbers.
While we've never been best friends and we didn't do anything outside work, we got along pretty well most of the time. So a few months ago, I happened to see her on a newspaper of sorts and I thought I should send her a photo of it and lightly make fun of her (neither of us takes the other very seriously, so no worries). We ended up talking a bit and bringing each other up to speed on what we've been doing the last five years and we ended the conversation cordially.
Then, on my birthday, she sent me good wishes, which... surprised me. I didn't think she would still have my birthday written down anywhere as she didn't write to me the last five years before I contacted her. It was a pleasant surprise, but now... I'm not sure what to do. I have no idea when her birthday is! I looked everywhere and I just can't find it. I don't really want the good wishes to be one way, but I feel it would be kinda awkward for me to simply ask her when her birthday is.
How can I tactfully find out my friend's birthday when I can only contact her over text?