well over fertilizing usually looks like a burn on the tips and edges of new and old leaves. In hydroponics between 600-800ppm of nutrients is good while vegetating, and in flowering I use no higher than 1200 ppm.
edit: I should mention, the more light you have, the more nutrients the plant will accept. A plant under a dim light grows slower and therefor needs less nutrients, and a plant under a very bright light will grow faster, and require more. Of course go on your plants response.
Under fertilizing comes in 2 categories, major (N,P,K, MG, Ca) and minor (micro nutrients).
*pulls out plant production guide...*
major nutrient deficiencies of NPK and magnesium will show on older leaves first, by looking dull (low phosphorus), yellow (low nitrogen), dead spots or leaves (potassium), or odd yellowing starting near the tips on leaves (magnesium), small yellow spots and slowed or halted growth (calcium).
minor or micro nutrient deficiencies will appear on new leaves, the most common is Iron (yellowing between leaf veins), less common is copper (new leaves get smaller and smaller), manganese (similar to iron deficiency), sulfur (yellowing of new leaves), and some more.
in any case if you're having trouble with overfeeding flush/change the reservoir with a milder concentration. if your having trouble with a deficiency, address it as soon as possible, if the problem is on new leaves, its micro nutrients, old leaves it's major nutrients. Check the nutrients you are using and be sure they contain a full *diet*, ask for suggestions, do research etc. one more thing, Mind the ph, for if the ph is too far one way or another the plant can't absorb what it needs (ph of 6 generally is good).
One year I had a problem with calcium deficiency and was constantly fighting it by adding a cal-mag, soon after I switched from RO water to plain tap water, and haven't had the same issue. So I guess what im trying to say is look at the plant water as a whole, for not one thing, but the sum of all things in the water determines how well the plant will react.