I just got yamaha R125 but I think mileage was chaneged in it, how can I check if there was any change done to the bike?

First thing to do is compare title to odometer other than that there isn't many other way except by wear and guessing... But you can't take that to court....Good Luck!

Check the sprockets and chain they will be a good indicator of how much original mileage has been put on it....if teeth are worn chipped curved or bent or come to a sharp point and you can lift the chain off the back sprocket and see daylight between the two them...then for a 2008... its probably got at least 10 to 15k mileage on it.

Can you not check your mot/service history.
If not phone/email D.V.L.A. They should probably know I think.