I'm not sure exactly what you're asking, but there are probably an untold number of factors when analyzing risk on sovereign debt. Probably amongst the forefront would be history of payment on bonds, currency stability, GDP, tax collection, and government stability.

For instance, I wouldnt buy an Argentine bond - they've failed to make bond payments to such an extent they had a warship seized as collateral in a foreign port. Likewise, Greece has had trouble making payments. GDP is poor, and tax evasion rates are high. To boot, because Greece is on the Euro, it cant even devalue its currency to lessen its debt burden.