Hello, IrishGardenGirl. At this time of the year, I would expect hydrangeas in the Northern Hemisphere to start shutting down until Spring. Some of that entails leaves turning -eventually- yellow, red, brown colors, then drying out and falling off.
Green leaves that show browning at the edges is typically insufficient soil moisture but some tiny pests can also nibble there so inspect the leaves to see if you have visitors.
You can use the finger method to control soil moisture: insert a finger into the soil to see if the soil feels dry, moist or wet. Then water if it feels dry or almost dry. If it feels moist or wet, skip watering. Another way to keep up with watering is to use a soil moisture meter. I bought one several years and misplaced it. Then I bought another one last year and misplaced it again. Lord! When I finally move to another house, I bet someone will find them! Ha!
As production of chlorophyll is reduced in the Fall and Winter, some leaf fungal infections that were previously present (but were also almost invisible) now become obvious. These look like brown spots but they can also have yellow or orange and purple-ish tones. Overhead watering is typically the cause of these. Watering only the soil and never the leaves is the easiest thing to do to prevent them but once a plant gets it, you cannot get rid of completely. See cercospora leaf spot in the link below: Rust looks similar to cercospora but the rust spots tend to be smaller and more orange.
Alkalinity can be tested in one of several ways. You can use some of those test strips used in school or you can purchase a kit at a local plant nursery to determine if the soil is acidic or alkaline. It is a pain but needed in some places like mine or when you want blues in alkaline soil. I usually determine about how often to acidify my soil and then use Electronic Reminders with Microsoft Outlook to tell me when to add more amendments.
Once the plants go dormant, feel free to leave them in a garage or shed where you can water them once a week or once every 2 weeks until they leaf out in Spring. Hydrangeas do not do well inside homes, long term wise. But it is ok to keep them there if you do not have a garage or shed, just keep them away from drying windy areas (like near heater or a/c vents in the ceilings or the floors)..