How am I supposed to find out who they belong to

It's quite easy ... every sheep or goat has to have ear tags by law ...

These identify where it comes from ...

Catch one and read off the holding number (the first part of the code) and
flock number ... you get a different flock number for each type of animal -
I have a separate number for my sheep and my poultry, but the same base
holidng number.

Phone up you local agricultural office - e.g. in Scotland its SERAD or the
local State Veterinary Service office and let them know the number and thye
will be able to point you in the right direction. If you are in England, the
local equivalent of SERAD is DEFRA.

All you have to do is catch one and note down the tag !

Top tip - go for one with horns, they are easier to handle.