Does it smell noticeably different than when you first opened it, or is the scent pretty much the same?
Your bottle should be totally fine at 8 months. Perfume is legitimately stable stuff - I had a fragrance sitting around for years that still smelled identical to when I bought it. The real issue is that bathroom storage is genuinely working against you. Heat and humidity from showers create temperature swings that actually break down the fragrance faster than age alone would. It's not going to ruin it overnight, but over time that environment does damage the delicate compounds.
If you want to keep it fresher longer, move it somewhere cool and dry - a bedroom closet, a drawer away from windows, that kind of thing. Direct sunlight is your enemy too, so anything opaque or dark is better. As for telling if it's actually gone bad, if the color looks noticeably darker or the smell seems thin or off compared to how you remember it, that's a sign degradation is happening. But honestly, 8 months in a bathroom? You're probably still good. Just swap the storage spot and you'll get way more life out of it.