There's different types of nose jobs... Some are simply shaving down a bump in the nose bridge to slimming a bulbous nose to inserting an implant to define and raise the nose. The latter is the easiest one to identify and coincidentally is the most popular one as it achieves the biggest change.
There are a variations of implants that be molded and inserted to sit above the nose bridge. The one thing all these implants have in common though is that they are meant to be made in perfection... So perfect that they often show on the nose as a perfect rectangle.
Sides are perfectly straight and symmetrical.
Transparency is another indicator. As an implant is inserted, skin is lifted and stretched and produces an unnatural sheen that almost looks like oil except it is perfectly symmetrical and runs evenly throughout. Keep in mind though, this can be covered with makeup.
In side view, a "line" can be seen on close observation. This is the implant.
In noses which have been slimmed and not given enough height compensate it, it will look like this:
Pinched. Even when smiling her nostrils stay unnaturally close together. The nostrils inside may have been sewn to bring them closer together. This is Dawn Yang by the way. From what I can see, not only did she fix several areas of her face... She's fixed her nose several times alone.
Her next nose job increased the bridge and tip to make the sides look more natural. Slim noses are usually also tall. Having a slim nose with a bridge that isn't high enough draws attentions to the unnatural look of the nostrils. The attention now is brought back to her bridge.
Still, the tip was too thin, leaving the nose looking awkwardly hollow. (Note the straight line again and transparency).
Her more recent nose job is probably one of her best, I'm wondering if she achieved this with fillers though. It gave her nose less of a hollow feel and compliments her face better now. However, this picture is from her blog which she photoshops (I can easily see in many pictures she blurs and erases part the top of her nostril out) so whether it really looks like this in real life is hard to say. The tip is widened and extended.
The ever so beautiful Angelababy. It pains me how gorgeous she is, lol. What do you think? Most of us already know how drastically different she looked in her younger days... So did she or didn't she (in terms of a nose implant, anyway)?
I would say no.
Her nose is not perfectly symmetrical nor does it run perfectly straight on both sides
Shadows are natural
Lines do not run all the way down straight to her tip. It flares out in the end.
Nor do I believe Gahi got a nose job for the same reasons.
Keep in mind, shine does not equate to translucency. Furthermore, contouring and photoshop can also make a nose look fake in matter of minutes. If all else fails, ask the person to push their nose up to imitate that of a pig if you dare. Implants can't bend, they're vulnerable to snapping. However, I don't think that their choice is really of your concern. But if you are curious, this is a general guideline. :)