How do you honestly know that your slowly going insane?

It is different for everyone and it depends on what you mean by insane.

I know my mental illness is getting worse because I lose control over my temper more frequently now. Once it is on there is no predicting how it'd end. Having more frequent episodes of complete incoherence and unstoppable rage. More memories getting distorted and lost.

You don't. I heard only sane people only question their sanity.

I think in all cases you will know. You will feel helpless and things won't make much sense to you. You will doubt yourself.

For it's emotions, usually love, where lol I feel like I'm losing touch with reality.

Insanity is doing the same thing over and over again and expecting a different outcome everytime

When everything seems to make sense

From the reaction of others. Because insanity really is your inability to function in society.

When everyone becomes normal

If I'm aware that I'm going insane then I haven't gotten there yet.

When you think everyone else is insane.