a sidelong means to look at something or someone by just moving your eyes to the side, without looking at it directly. Glance or glimpse means to look at something for very, very short time. A quick look. Now when you glance at someone sidelong, you might not want that person to notice that. In other words you can say this is to look at someone sneakingly or secretly. While you're glimpsing at someone or something, that means you're paying attention to that or looking directly even for a very short time.

You would use the phrase, "sidelong glance". A glimpse means that you just had a short view of someone. A sidelong glance is a flirtatious look.