The warning signs are probably different for everyone. For me, it's small noticeable differences in my patterns of thinking. Things for me tend to be the worse at night and this is the time I'm more likely to....

a. self harm
b. attempt suicide
c. become overwhelmed by emotions
d. send stupid emails to my therapist

Changes in my thinking are....

a. Thinking more and more about if I should self harm and how
b. Things begin to snowball in my mind (ex. I'm not where I want to be career-wise and I have to be secure in my career before I can think about a relationship and children, but I'm getting older and time is running out.....but there are all these obstacles because of my past and nothing is ever going to change so I should just kill myself.....so what do I need to do to get everything in order, should I just stop going to therapy and get off meds since there isn't any point now.....etc etc.)
c. It gets harder and harder to stop myself from those snowballing thoughts. First they start off harmlessly and I can bat them away, but more and more it becomes something I can't control.
d. I isolate even more.
e. I spend more time in bed.
f. I leave the house less.
g. I start to feel like I'm out of control.