While you can look at eh leaves and depending on what you see could tell you what the deficiency is. But many times it is not obvoise as to what the deficiency is. For example I did have a copper deficiency once and the only symptom I had was very slow plant growth. The plants looked normal.
One hing I found to be very helpful is to look at a list of the nutrients plant need and compare that to what is in your fertilizer.
The Needed Macro nutrients are Nitrogen (N), Phosphorous (P), potassium(K), Calcium (Ca), magnesium (Mg), and sulfur
The needed micro nutrients are iron (Fe), Boron (B), Chlorine (Cl) Manganese (Mn), Molybdenum (Mo), zinc (Zn), Copper(Cu), Nickel Ni)
I have GH, KH, Phosphate, and Calcium checkers coming in the mail tomorrow along with "Calcium Nitrate Ca[NO3]2" and a TDS meter came in the mail today.
By "Checker" are you referring to the Hanna instruments Colorimeters? If so be advised that my understanding is that the calcium checker is for salt water only. Hanna does sell a calcium hardness and magnesium hardness colorimeters but they are intended for lab use and have a maximum reading limit of about 1ppm. Aquariums will always read off scale high. That said I do have the hanna phosphate, alkalinity, and ammonia Colorimeters and I like them alot.
Your TDS meter measures the water conductivity and compares that to a calibration reference. While you might see 300ppm on your meter someone else with a very similar aquarium may read somewhat higher or lower do to differences in the water chemistry. However even with this limitation TDS meters are very helpful. A TDS reading of 300 is considered Moderately hard according to this site:Hardness,Conductivity,TDS measurement
I have considered experimenting with Calcium nitrite but at present i have not done that yet.
if you then look what is in your fertilizer you will see some problems. Most people just dose NPK and don't think about Ca, Mg, and sulfur. Chlorine and nickel are also missing.
Since your fertilizers are providing everything but Ca, S, Cl, Ni the most likely deficiencies are Ca, S, Cl, Ni. Now tap water typically has more than enough l Cl. The amount of nickel needed by plants is so low that tap water and fish food should have enough. Calcium and sulfur are however are macro nutrients and plants need a lot of these. As a result these are in my opinion the most likely to cause people problems. On good way to test for a calcium or Sulfur deficiency is to is to try a sulfate GH booster such as Sachem Equilibrium. It contains calcium sulfate which would deal with both potential deficiencies at once. Or you can just by calcium sulfate apply some to your tank.
Unfortunately these fertilizer deficiencies are very common. I have searched but so far have not found one that has all of the needed nutrients. Ca, S, Cl are often not added because they are common in soil on dry ground or are typically present in tap water. Foster fertilizers were not designed for aquatic plant and aquariums. Additionally with CO2 injection and high light aquatic plants will burn through nutrients very fast which can make it hard for you to keep up with the nutrient demand.
I would like to provide you a good link but much of the above Is what I have learned on my own from my own experience and observations on aquarium web sites. Web information write and wrong and sometimes can be misleading.