Depends what you're asking?
If it's marriage or moving or in, don't ask! If there is that level of doubt, your relationship it just isn't at that stage yet.
If it's going out on a date, just ask. You can't lose what you don't have. If she says no then you are in the same position as you are now, a guy with no-one to love!
If it's what @It's Your Round is suggesting, just ram it in, guage her reaction and if it's negative, immediately apologise and plead some kind of accident!