¿Se lo dijiste?

¿Se lo dijiste?
 

This is just an example, but how do I know if it's translated as "Did you tell them?" or "Did you tell him?"

By the way, Happy Easter. :D

I'd go with this,

¿Se lo dijiste a ellos/ellas?

¿Se lo dijiste a él?

Yes, Happy belated Easter (it was 2 Sundays ago... don't you remember?) (Today is a very important celebration in China!)

I'm a Muslim. :D
The kids started searching for eggs and got presents yesterday. So, I thought it was yesterday.

Thank you for the answer. Strangely, the subtitle was "¿Se lo dijiste?" and meant "Did you tell them?".

Only context will tell you whom the sentence is talking about. Isolated like that, it's definitely ambiguous. That is why Pablo suggested adding the pronouns. :)

(By the way, I said "it was 2 Sundays ago, because today is April's Fools!)

It may help if someone mentions that 'lo' is a direct object pronoun, taking the place of what was said to him/her/you/it/them.
The third-person indirect object pronoun 'se', which replaces 'le' or 'les' when there is also a third-person direct object pronoun, is ambiguous.

I totally forgot that. :)