A few questions. Ph, light issues and whiteflys

How will I know if I have a ph problem. Like, what signs will I see if it's low or high?

My neighbor has had a small light on for a few weeks. What would I look for if it was causing an issue.

Whiteflies. How much of a problem are they? I'm 5 weeks into flowering. One of my six plants usually has 10-20 fly around when I kick my plants pot.

A. Get a ph test kit
B. if it's outdoors and not too bright it shouldn't cause a huge problem.
C. A big problem. Check the bug forums for some good ways to get rid of them quick and safely, i'm not sure how that deep into flowering.

I know how to test ph. I mean what happens when it's low or high. My plants seem awesome so either it's spot on or if it's low or high, my plants are enjoying it still.