The particle ли is put behind the verb, but the verb to be is omitted in present tense. If you want to say something like "How do we know this is true?", how do you translate it to Russian? "Как знать, это ли правда?" ?
Как знать, это ли правда?
First of all, "как узнать". "Как знать" is valid but means doubt: "it's impossible to know". Why? Because you can't "знать" before you "узнать" and you're not doing that.
Как узнать, это ли правда?
Second, this is a valid form but rare, and it's a good translation. It means that there are several probable truths and you try to find "the one". And you're asking "Is this one true?"
If you just have something that is true or false, you will say:
Как узнать, правда ли это?
These are two valid uses of ли. So obviously you need to put
You can also put
ли after both words in speech, but never before both words. I don't know why.
This is a complex sentence with a yes/no question in a subordinate clause. We usually put an interrogative conjunction "ли" after the meaningful word, which bears the main information in the sentence, so the most common variant would be
Как знать (узнать), правда ли это?
You don't know if it's true or not, you want to know that, the stress is on the word "правда".
Как знать(узнать),это ли правда.
It is possible, but rarely used. Here the stress is on "это".You want to know if this specific information is true from several facts. This isn't a usual construction. We use it only in specific situations.
Ли is Wackernagel's clitic, which means it goes after the first fully-stressed word of the sentence (not after the verb, as you say). It tends to move before itself the "question's focus" - if no specific focus is given, it's usually the predicate.
Как мы узнаем, пришёл ли Вася? - How shall we know if Vasya came?
Как мы узнаем, Вася ли пришёл? - How shall we know if it was Vasya who came?
Regarding your question, one should say "Откуда нам знать, правда ли это?" (More literally - "Как нам знать, правда ли это?", but "откуда" (where from) sounds better).