Dylan - a lot of views and no responses usually means we are stumped. The “25” is all I can see of the mark on the mirror, and if it were “925” and not “25” it would probably denote sterling silver. The almost universal rule of thumb is that if it is solid silver, it will be so marked, for example if from the UK, with the legal hallmarks, if American, with the word “sterling” or “925”, if from Europe, a country’s silver mark, etc. If it is not marked solid silver, it is almost certainly silver plate or another metal such as pewter.
Try a simple trick I was shown years ago: briskly rub a flat surface of the mystery metal with your thumb to heat it by friction, then smell the surface. An acrid smell is silver plate where little or no smell is silver or pewter. Most silver plate is a very thin layer of silver over copper and heating the copper base will produce the acrid smell.
But 99+% of the time, no silver mark, not silver.