Better safe than sorry? Should you confess if you're not sure if you've committed a sin?

1 JN 1:9 If we confess our sins, He is faithful and righteous to forgive us our sins and to cleanse us from all unrighteousness.

The book of 1st John in which the verse is pulled from above is written to believers instructing them how to have fellowship with God. It's is NOT instructing them on how to have eternal life. Eternal life is the main subject of the gospel of John. The requirement for eternal life is belief in Jesus and not confession.

16 “For God so loved the world, that He gave His only begotten Son, that whoever believes in Him shall not perish, but have eternal life. John 3:16

Notice that is does not say, whoever believes and confesses. The Bible also claims that we all too often sin without being aware that we are sinning.

Lev. 5:17 “Now if a person sins and does any of the things which the LORD has commanded not to be done, though he was unaware, still he is guilty and shall bear his punishment.

I don't think it's possible to confess all of the sins we'll ever commit. God's solution to the problem of sin is not confession but a payment in the body of Jesus that is applied to our account at the moment of faith in Him.

Concealing sins—especially mortal sin, of which you seem to be speaking, because dying in the state of mortal sin means you'd go to hell—is itself a serious sin:

Catechism of the Council of Trent, chapter on penance:

CONFESSION SHOULD BE ENTIRE

[…] All mortal sins must be revealed to the priest. Venial sins, which do not separate us from the grace of God, and into which we frequently fall, although they may be usefully confessed, as the experience of the pious proves, may be omitted without sin, and expiated by a variety of other means.

[…]

SINS CONCEALED

So important is it that Confession be entire that if the penitent confesses only some of his sins and willfully neglects to accuse himself of others which should be confessed, he not only does not profit by his Confession, but involves himself in new guilt. Such an enumeration of sins cannot be called sacramental Confession; on the contrary, the penitent must repeat his Confession, not omitting to accuse himself of having, under the semblance of Confession, profaned the sanctity of the Sacrament.

I will try to draw a simple line under all this - to put it simple - even if you're unsure whether or not you committed a sin, you should confess it and I see at least 2 reasons for that - 1) You are having doubts about your actions, therefore for you as a believer natural action is to see priest - he will determine whether you committed a sin or not, also as a believer you have to clear yourself of any doubts regarding your Heavenly Father - that is of course only, if you truly believe. 2) Hiding actions that could've been sin, you immediately commit another sin by not confessing them and again - you come face to face with question - do you truly believe? - if answer is yes, then there shouldn't be any doubts, fear or even thoughts to hide something.

I apologize if my wording appears a bit "sharp-spiked" to somebody.

If you aren’t sure if you have committed a sin it is better to confess anyway?

If the action or deed may be a possible venial sin then one may go to Confession. It will not do any harm and one may even get some spiritual insight into leading a holy life.

If however one is not sure if one has committed a mortal sin, one should go confession. Mortal sin for a Catholic must be confessed to a priest within the Sacrament of Reconciliation.

Mortal sin destroys charity in the heart of man by a grave violation of God's law; it turns man away from God, who is his ultimate end and his beatitude, by preferring an inferior good to him.

Venial sin allows charity to subsist, even though it offends and wounds it. - Catechism of the Catholic Church.

If in doubt, it would be best to go to confession. If one did commit a mortal sin, one would be restored to the state of grace and the sacramental life within the Church. If one did not do a mortal sin, then one has gained insight into leading a holier life and obtained confidence in going to a priest in the Sacrament of Confession as means to keep oneself holy.

Should you confess if you're not sure if you've committed a sin?

Your question can be understood in two different ways:

1) You did something you definitely see as sin but you don't remember that

When I was young I was told that after the confession sins are also forgiven if you don't remember them and therefore did not confess them.

2) You remember well what you did but you don't know if it's a sin

You are only forgiven sins after a confession if you are sorry about what you did. So if you are not sorry about having done something, it makes no sense to confess that.

... it is better to confess anyways than to be stuck in hell forever?

Not "the" catholic perspective, but something I have read in a text written by a Roman Catholic bishop from the Southern American continent; the bishop wrote something like this (I don't remember the exact words):

Often I had to do with people who wanted to confess before they die.

Of course they wanted to confess because they wanted to protect themselves from God.

I think that it is a sin to think that it is necessary to protect yourself from God.

(My personal opinion when I was reading this was:

"... and to think that it is possible at all to protect yourself from God.")