In speech we have the possibility to emphasize the meaning. Stress Das and sustain wohl a bit to signify intensification and your accordance.
In written text it is hard to avoid misunderstanding, though you can indicate the connotation in the sentences before and after. I think in your example the reader would assume the second variation, because - imho - it is more common.
If you have a sentence in which it is hard to differentiate the connotation, you have two possibilities to clarify the meaning
You replace wohl
Das ist wahrscheinlich wahr. (probably)
Das ist durchaus wahr. (true)
or rephrase the sentence.
Das wird wohl so sein. (obviously probably)
Das kann man wohl sagen. (obviously true)
Regarding the translation there are three scenarios
You are able to get the meaning from the surrounding text.
"Das ist wohl wahr, aber ich glaube es nicht wirklich."
But I don't believe it alludes to probably. In this case it is easy to translate it.
It's much more complicated, if the surrounding text still don't clarify the connotation.
"Das ist wohl wahr, aber sicherlich nur wenn man ... ."
Then you have to go with possibility 2 or 3.
Use the word which is more likely or common in context.
- Leave it out if the word is negligible.
But note: In your example it is necessary if you want to emphasize probably, because That's true indicates the second variation.
It's hardly possible to translate a word like wohl if the context does not clarify its meaning. I propose to leave it out if possible. If you get the feeling leaving out gives the wrong connotation then add the more likely word.