Planted Tank Guru
Join Date: Jun 2004
Location: Baton Rouge, LA
The diagnostic list Jargonchipmunk linked to is pretty complete, and I've had it saved on my computer for a while.
But in practice, the three most common symptoms and causes in my experience are:
1) Plants growing excessively tall with sparse foliage. Caused by too little light.
2) Leaves coming in yellow and never achieving proper green coloration. Caused by too little iron.
3) Older, lower leaves on broad leaf plants developing pinholes, which get larger over time. Caused by too little potassium.
Plants rarely in my experience show signs of excess light/nutrients. However, other things do. Excess light will cause algae. Excess CO2 will cause fish to gasp at the surface.
And other nutrient deficiencies may not show specific symptoms, but instead weaken plants or slow their growth; leading to algae accumulating on or parasitizing them. Some specific algae prefer conditions of deficiency even if they don't grow on plants.
This is a very broad topic, but this is the most relevant info I can give you in brief to get you started. Hopefully it will help. Let us know if you need more details in a particular area.