Hard water could be damaging your hair if you notice one or more of the following:
1. Your hair gets very tangled after washing
2. Your colour-treated hair fades easily or turns brass-like and dull quite rapidly
3. Your hair's breaking and nothing appears to be able to stop it
4. Your hair is highly unresponsive to treatments, e.g. if you relax your hair it doesn't even straighten well
5. Your shampoo doesn't lather. Note that some natural shampoos don't lather anyway so if it's a new shampoo that isn't lathering, that could also be the issue
6. Your hair's always dry and it just won't respond to anything
You might also like:
The 3 best ways to protect your hair from hard water!
The Science of Black Hair, pages 73-5
Hard Water Hair Damage?
Hair Treatment for Hard Water