Yes you can over feed plants, it normally shows up as discolouring of leaves, bascically the plant is being burnt, personally I use a season long feed, Vitax Q4, these are granules mixed in with the compost. This will last all the growing season normally.

Sadly, I have done it myself! Too much nitrogen usually results in the new growth turning black and shriveling. Older foliage may fall off. Too much phosphate will cause chlorosis, much like iron deficiency. Too much potash will cause symptoms like salt burn: brown tips and edges to the leaves, with a narrow yellow band just inside. Confusingly, too much micronutrients (iron, zinc, manganese, etc.) will usually cause symptoms identical to deficiencies of thes elements!

I am always feeding my garden to keep it at its best, but always worry about overfeeding which would have the opposite effect to that of which I desired.

Best to follow the instructions on the bottle or packet.

And, when in doubt, remember that while too little food slows plants down, too much can kill them!