I want to know if my cassette is worn. How can I tell as when I change gear it looks like the chain rides ontop of the sprockets rather than connect with them properly. I had to change the chain not long ago if that helps.
If you waited too long before replacing your chain, your cassette needs replacement. The cogs on your cassette wear with your chain, and as your chain stretches, the valleys between the teeth of a cog will widen. Your most-frequently used cogs will exhibit the most wear. If you have recently replaced your chain with the correct spare, and this is a new problem, then replace your cassette with a new one. Keep your new chain clean and lubricated, and monitor chain wear with a quality chain checker. If the chain wears to stretch greater than 1% of it’s overall length, replace.
A crude way to judge the wear on cogs and chains is to grasp the chain at the back of the cog and attempt to pull it backwards. The distance you can pull it away from the cog is a measure of chain and/or cog wear (depending on which has more wear).
This works even better when judging chainring wear.