We all have different goals here (whether it is just to tone and even skin out, remove permatans, get a bit lighter or drastically lighter). However, no one wants that "bleacher" look (ie. Sammy Sosa).
Please use this thread to list some of the obvious signs of bleaching and how to avoid them.
A few off the top of my head:
Awkwardly uneven tone. For example a bright face but dark neck, ears, perimeter of hair line. This can easily be avoided by not neglecting other parts of your skin during skin care.
Dark joints (knuckles, knees, elbows etc). This isn't always a telltale sign of bleaching, as joints on naturally lighter people can also be dark but coped with other signs it can add up to scream "bleacher".
Very darker lips. This is also one that isn't a sign on its own but can contribute to help others come to the conclusion.
Darkening very fast in the sun. I went on a beach date with a naturally light skinned friend and a artificially light skinned cousin and let's just say, people who lighten darken much faster in the sun. I wish I could give advice on how to darken at a slower pace such as a naturally light person but I don't know. Use a high spf sunblock always.
Discolorations are darker on those who lighten. When my naturally light skin friend scars or gets a discoloration from a popped pimple it tends to be a light brown. However, when I see someone who has lightened their skin drastically get a discoloration, it is much darker.
Greyish, dull undertoned. I've noticed that naturally lighter people have richer undertones, whether it be red or yellow etc. Having this type of undertone also makes your skin look drastically different in different lighting (more so than those who are naturally light). As another poster said, when you are artificially light, you can easily appear dark skin in different lighting when in fact you are very pale. I've seen this effect in photos next to naturally light skin individual in different lighting. When I was lightening more heavily, in sunlight I was lighter than my naturally light friends, in dark photos I was darker. Ways to combat this are lightening from the inside. Taking supplements such as vitamin C, L Gluta, drinking tons of water etc.
Thin skin. This may be in result of over exfoliating or using products with steroids. This can be avoided by not using products with steriod, not over exfoliating and building skin using products with skin building ingredients such as collagen and vitamin E and A.
Skin color tends to fluctuate way more than the average person. Yes we all can tan and get lighter depending on the weather. However, I've noticed those who lighten their skin have way more fluctuations in skin tone regardless of the weather. This can be avoided by coming up with a maintenance regimen and sticking to it. Try coming up with a regimen for all 4 seasons, as what works for summer may over lighten you in the winter.
Please feel free to add one telltale sides and tips on how to avoid them. Feel free to add more tips to the signs I posted above, as I feel I did not include enough tips.