It really shouldn't be hard to tell for the designer. If you're considering implementing loops, conditionals or variables in your DSL then you should kind of realize what direction you're headed in.

The hard part is realizing from the get go (before backwards compatibility concerns kick in), that your problem space is not conducive to non-turing complete languages in the first place, and that instead of inventing an exciting new DSL, maybe you should just write a library.