IMO Bitcoin's limited amount incentivises speculation/hoarding over spending. Dogecoin, on the other hand, with its inflationary approach feels much more like something that can be used as a currency.
I sometimes estimate (3) and (6) by monitoring news stories. It doesn't give me absolute numbers, but some sense of scale. I sometimes see values reported for (2) or (4) from individual merchants, but they are rarely useful: the merchants report the great response they had in the weekend after first announcing their support for bitcoin, then we never hear numbers again. Occasionally we'll hear from sites like reddit about the total amount of contributions (not exactly goods, but close) being made in bitcoins: these are usually quite disappointing. I can think of no way to monitor (1) and (5) is likely kept secret on purpose.
So these are the fundamentals I care about and that I think affect the long-term value of bitcoin (as opposed to its short-term speculative value). But I don't have a way to measure them, except for guessing based on press coverage.
2. Number of users SPENDING bitcoins to purchase goods
3. Number of merchants accepting bitcoins for goods
4. Total $ value of bitcoins being spent to purchase goods
5. Total $ value of bitcoins being spent to perform international money transfers
6. Amount of press (bad press OK, good press is worth much more) that bitcoins are getting as it drives 1 and 3.
I'm sure others have their own list of fundamentals.