How to tell deficiency

[Colorblizzle]

Ok so instead of going the dry ferts route I got flourish instead. I'm gonna dose half of what they say to use to start since its a low tech tank and I want slow steady growth BUT my question is how do I know what I need? I don't know what minerals and stuff are already in my city water. I bought NPK and trace from seachem but if I have as an example potassium in my water Already I don't wanna overdose. I've tried contacting my BPW but can't get through to the right people. Any fool proof way to tell if I need all 4 things? Planning on dosing once a week