Determine if in paint or crack?

[tuscanyswe]

Hi

So i have this nice frame with what potentially could be a crack.
This was a very thin line from the beginning, tried to chip bit of with a screwdriver but dident reveal much more.

How do i go about determining this in the best way?
How much paint do i need to remove around the "hairline"?
And how do i best remove it?
Do i need to use a steel brush in the end or similar to c properly?

Thanks for your suggestions.