The Die I break is in the left inner frame between the shading line that aligns with the top of the words of value and the line above that.See this halfpenny roo for a good example
Some stamps such as the First Watermark One Shilling roo you refer to above only exist as Die II, so that makes the job easier.
Where stamps of the same watermark exist as both Die I and Die II, the Die I break is the determining factor. If the break is unclear due to a particularly poor or blotchy print then, apart from shades that might only exist for a particular die, the other means is to identify any plate varieties (flaws) or characteristics associated with the die.
The flaws are listed in specialist literature such as the BW ACSC Kangaroos catalogue.
Some stamps such as the 3d kangaroos have defining characteristics to help separate the dies - e.g. teardrop shape ball at the top of the numeral 3 on the much rarer die II 3d (in addition to lack of die I break and without die IIB characteristics).