What should the Euro regulators look for and monitor to prevent a Euro crisis?
For an EU member state to joins and stays in the Eurozone (the official name of what you called the Euro Club), it is necessary to comply with the Stability and Growth Pact (which Bulgaria already does) and fulfill the Euro Convergence Criteria defined in the Maastricht treaty article 140.
In a nutshell, the convergence criteria define targets for the country on inflation, public debt deficit, interest rates and exchange rate against the Euro. These targets should be followed for a certain duration of time.
All of these targets were put in place to monitor and prevent economic instability - and after the debt crisis in several member states (such as Greece) these indicators keep on being used as good indicators to monitor the stability of a country's economy. However, preventing a economic crisis would require a higher integration of the member states, such as a Euro-zone public budget.