Watch ye therefore, and pray always, that ye may be accounted worthy
to escape all these things that shall come to pass, and to stand before the Son of
man. - Luke 21:36

Often there are no signs at all. In my own experience the symptoms to look for are unforgiveness and selfish behavior. The National Institute of Mental Health gives the following list of behaviors as a warning sign for suicide.
If you know someone that you believe to be suicidal, you must watch that person 24/7. Unfortunately, most people are unable or unwilling to make this sacrifice for another person. Our society has conditioned us to turn this responsibility over to the professionals. Unfortunately, the professionals will usually medicate their patient.
All professionally prescribed medication for treating depression have side effects, including causing suicidal feelings . At least have your doctor explain all the side-effects of your medication (and the side-effects of withdrawal) before you start taking it.
A time to weep, and a time to laugh; a time to
mourn, and a time to dance
- Ecclesiastes 3:4
It is important to understand that going through depression from time to time is a normal part of the human condition. Feeling suicidal does not last forever. When you are depressed, you must not give up hope. There are very few people who have not at one time or another had the same struggle.
There hath no temptation taken you but such as is common to man: but God is faithful, who will not suffer you to be tempted above that ye are able; but will with the temptation also make a way to escape, that ye may be able to bear it. 1 Corinthians 10:13
Committing your life to Jesus Christ, is not the same thing as saying the sinner’s prayer. Often we have to come to the point of despair before we reach out to God. Many, including myself, have found deliverance from depression and other mental disorders simply by giving their lives to Jesus. The ancient King Nebuchadnezzar tells his own story:
The same hour was the thing fulfilled upon Nebuchadnezzar: and he was driven from men, and did eat grass as oxen, and his body was wet with the dew of heaven, till his hairs were grown like eagles' feathers, and his nails like birds' claws. And at the end of the days [seven years of stubbornness] I Nebuchadnezzar lifted up mine eyes unto heaven, and mine understanding returned unto me, and I blessed the most High, and I praised and honored him that liveth forever, whose dominion is an everlasting dominion, and his kingdom is from generation to generation: Daniel 4:33-34
While many put off or refuse the offer of Christ because they feel they would have to quit having fun, the truth is that they are miserable and if they would give their life to Jesus, to do with as He pleased, they would have our common experience.
Yet I will rejoice in the LORD, I will joy in the God of my salvation. - Habakkuk 3:18