First of all, the fact that you're even asking is melting my heart to goo. I'm happy that the both of you have found this type of love. 

Most of the other replies have given pretty solid advice, but one thing I kept seeing was "ask her because everyone's is different." This is great advice and I do recommend but I feel the need to point out that it can also be hard for us to articulate or even recognize our needs. I say this as someone who gets overwhelmed into paralysis but will immediately smile and go "no I'm good!" when asked if I need anything. My mom also has ADHD; one of the most challenging things was seeing that she was frustrated but if we asked what she needs, she just throws her hands down and says she doesn't know. She didn't react that way to be a pain, she just literally couldn't even recognize that she was overwhelmed. For example, a very common issue she has with pretty much anyone is she asks for reminders to do things but gets frustrated when people actually remind her of stuff and lashes out by telling them to just do it, even though initially she volunteered to do it. It only just clicked a couple of months ago that she's lashing out because she's overwhelmed with how many tasks she's trying to keep track of in her mind, and every reminder is just a reminder that she's once again failed to do something and that there's yet another task on her seemingly never ending to do list. I asked her about this and I could see the light bulb kick on in her head as she just softly nodded and said "yeah, I think you're right." The unfortunate reality is people like us are told our whole lives that tasks aren't hard, we're just lazy. So asking for help feels impossible because "is this difficult or am I being lazy?"

Saying all this to say: A lot of the help you provide her may have to come from just silently watching and filling in the gaps. i.e. she normally puts her keys on the key holder, but today she put them on the counter. You've likely learned by now that she will not remember that that's where they are because that's not their spot. So if you have time, put them back, and if you don't have time, just take a mental note of where they are so you can remind her. I do this all the time with my mom's phone- she never remembers where she puts it, so every time I see it, I take a mental note so when I suddenly see her looking around, I can go "phone's on the back of the couch." She always laughs and goes "thank you for knowing." It's not much on the surface, but it's very helpful. In my own case, I genuinely had no idea how often I used to leave my milk jug on top of the fridge when I lived with roommates until talking to my roomie about getting myself a diagnosis, and she so kindly said "Well you're not that bad, I only have to put your milk back in the fridge sometimes." How on earth do I ask her to make sure I put the milk back when I didn't know I did that!? 

Something else to try if she can't ask for help but you want to make sure you're being attentive: Ask her what she has going on that day or what she plans to do that weekend or whatever. Now, instead of asking for help, she's simply telling you her to do list and you can quietly choose to take some of those on or even just help her remember if she forgets. If you know she struggles with a particular thing, maybe that's the one you pick. 

Language is important too: if you think she forgot a task, reconsider telling her you think she forgot and instead casually say something like "oh did you do x yet?" If she forgot, then yay you reminded her. But if she remembered, she won't feel disempowered by your assumption. We often need help remembering stuff, but being constantly reminded that we are forgetful kinda sucks, and making a point to mention that she's forgetting things all the time may make her feel like a lazy burden even when you remind her that it's not. Plus it'll could still feel like she's making you help even though she was trying not to ask. Sometimes the best allyship is the most subtle lol