By reading this, you are already following this recommendation: learn everything you can about OCD. That means learning about OCD generally and about the particular OCD patterns affecting the person close to you.
The more openly and curiously you explore OCD, the better you both become at recognising it. When the person with OCD begins to observe its patterns, that in itself is a helpful step.
Remember that the mental aspects are at least as important as the visible actions. Do not focus only on what the OCD is about or the arguments it uses. Look at how it works: the patterns it follows and the pressure it applies.
OCD can continue to find new subjects. When one source of doubt becomes less prominent, another may take its place. The new story can feel more real, more serious and more like something that must be investigated this time.
Do not become absorbed only in the content of the story. Look at what it is making the person do. Does it demand answers, checking, analysis, avoidance or repeated reassurance from you?
OCD follows patterns that can be recognised and, over time, anticipated.
OCD changes the story. Its way of working often stays the same.