As CX2 development is pogressing, it is not finalized whether CX1 translations will be supported in CX2 editing interface. A conservative approach might be to continue CX1 translation in CX1 editor itself and all new translations presented in CX2.
Whaterver be the case, there is no way to decide a translation was done in CX1 editor CX2. CX database does not save this information in database. Guessing based on the start of translation start date does not look like a reliable approach.

Proposal:

1. Add a new column to cx_translations table to store cx_version information. The values are number indicating the version.
2. When a new translation is started put the entry as 2 and load CX2 translation editor.
3. Not having a value in this column for a translation record or value as 1 means it was done in CX1 translation editor.