There's no right answer. It all depends on too many variables.
I prefer don't ask don't tell, but the closer you get to someone, the more you can kinda tell. I also never ask. I just observe and do my thing. Everyone on both sides is pretty aware of your communication, for example: if you text a lot in front of someone, they will get the sense it is someone else. if you call someone or text someone, and don't get answer for hours, between 7:00-10:00 you can kinda infer there is someone else.
In my experience, MANY women don't really like to fukk with more than one person. They may text or talk, but not date.
The times were I went out with like 4 people in one week, gets a little hectic. not random dates but staying the night sleeping with. You are getting texts from one on a Thursday, who you saw on Monday, and she's wondering, where would you rather be at 8:30, then at my house? and you are at someone else's. Def. been caught out with another woman, at a spot by, someone I was sleeping with the same week. She was mad, but she couldn't say I ever told her I wasn't. The "plausible deniability" gets thinner and thinner as you get closer and closer. but both parties often prefer to not openly ask.