I think modesty can be different for everyone; what may look modest on one girl can look immodest on another girl. A lot of it has to do with body type/shape. There are some things which would obviously be blatantly immodest for anyone to wear regardless of their build, but for many things it's a personal matter.
You need to see how that particular clothing looks on you and how you feel about wearing it. Is it too low, tight, or short? Would it come across as a stumbling block for men? If Jesus were standing right next to you, what would He think? Those are some questions you can ask yourself.
Modesty really is a matter of the heart; that's why God didn't give us specific rules and guidelines on what to wear, because a lot of it is dependent on your personal convictions and motives of the heart. You may be wearing something that you feel comfortable wearing and see nothing wrong with, whereas someone else does. It could be that it's a gray area; it's not necessarily wrong what you're wearing, but the other person sees it as that, because it's their personal conviction. But it could also be that perhaps what you're wearing is something indecent and we should listen to what the other person has to say and take heed to their advice. If you're not sure, just pray about it, and ask yourself those questions I mentioned above.
Sometimes modesty isn't what we're wearing, but how we're wearing it. They could be very decent clothes, but are worn in a flaunting provocative way. Perhaps the clothes scream "Look at me, I want attention." Modesty can also be seen in the way a person walks, talks, and carries themselves.
Modesty can also go for men too. If a man is egotistical, proud, and arrogant, he's not modest. And as far as how they dress as well. Men posing without a shirt on, showing off their physique and their pants so low you can start to see an area that you don't want to look at, certainly isn't modest!
So really, it all comes down to wear your heart is and if you're trying to follow God's way. If you seek after Him, He'll guide you in the right direction.