Full Idea
For Spinoza, the intrinsic mark of truth is the property which he calls 'adequacy'.
Gist of Idea
For Spinoza, 'adequacy' is the intrinsic mark of truth
Source
report of Baruch de Spinoza (The Ethics [1675]) by Roger Scruton - Short History of Modern Philosophy §5.6
Book Reference
Scruton,Roger: 'A Short History of Modern Philosophy' [ARK 1985], p.57
A Reaction
This is presumably the sort of theory to which early rationalists were confined, and it seems to be no advance on Descartes' 'clear and distinct conceptions'. I take it that the coherence theory is a better account of what they were after.