[Ju gatsu mikka]

The best solution to be absolutely sure that a portal is your is to build one one the correct coordinate inside the nether, light it, then extinguish the one on the overworld and then relight it.

When you light a nether portal on the overworld, the game will search if there is an existing portal within a certain radius around the corresponding coordinate and connect to it. If not, then it will build one in the nearest cave. By building yourself the portal inside the nether, then you are positively sure it's yours.

Here is a link to a nether portal coordinate calculator.