... how can I tell a decent quality saddle from a really cheap one?
Before you go to the tack store, ask your friend if she has any saddle type preferences, or even if she can go with you. We may tell you to buy an All-Purpose, she may tell you to get a dressage or close contact. She could probably also help with general suggestions as to the size of the saddle for you (17", 17.5", etc.) as well as your horse (tree sizes).
Are you looking for a used saddle or a new saddle? If you're looking for a new saddle, you can probably do some research online before you go and see what places like Dover are selling them for. The same goes for stirrup leathers, irons, etc. With new saddles, you pretty much get what you pay for. (Exceptions to this would be synthetics, of course.)
If you're looking for used saddles, go to the tack store and see what they have. Check the seat for cuts/abrasions, cracks. Look underneath the saddle at the billet straps and see if they're frayed, split or look damaged.
Make notes on any saddles you're fond of -- brand, size(s), overall condition. Then you can look on eBay, used English saddle shops online and make some comparisons. I'm not saying buy your saddle online, but look around and get a feel for what similar saddles and brands are going for.