How do I determine if my mussels are fresh?
Assuming your mussels came from clean, unpolluted waters, there is mainly only one thing that determines the quality of mussels:  Freshness.  

For all shellfish, this means alive when you buy them and alive when you cook them. Their shells should be tightly closed when you buy them, or they should close when you tap them.  

Mushiness doesn't mean much.  Shellfish are kind of mushy in their most perfect condition.