So... gotta work on your subtlety game.... there are certainly ways to ascertain if she is not smelling good. Finger her first. Then, somehow get it close to your face. Shouldn't be hard. Or just commit and go for it. Generally speaking... a woman's vagina is self cleaning. Get her wet, and it start to get better almost immediately. Another trick... if you get down there and it's not great, use lots of saliva. Again, kind of cleans things and it'll start tasting and smelling better (and she'll probably enjoy the extra wetness).
The one thing I would not do..... don't act surprised or disgusted if it's off. Suck it up and go with it for a bit. Or if you suspect an issue, kiss her thighs, moving up closer to her pussy. Kiss around her. Kiss her pubic bone... and keep moving up. Belly button, rib cage, nipples, mouth... and you are out of danger. Haha. Just whatever you do... don't make her feel bad.