Is it possible to view which artists are available on Ubuntu One music service without subscribing first?
You don't need to make any monetary subscription to access the Ubuntu One music store. All you need is a free Ubuntu One account (of course, if you want to purchase any songs on the store you'll need a credit card).
The range of music available will depend on where in the world you're located. The range should be pretty good if you are in the US, UK or most of Europe. Outside of those regions the range is a lot smaller, lacking most of the content from major labels.
The subscription music streaming plan is separate from the music store, and lets you stream any music you place in your Ubuntu One storage to a mobile phone. It can stream songs you've bought from the music store, but can just as easily stream songs you've ripped from CDs locally.
Looking at it from the other end, while you can stream purchased music using the subscription plan, the subscription is not necessary to enjoy the purchased music on the desktop (or any other system you copy the purchased music to: the purchases are DRM free).