If it says just 'कल या परसों', you have no way of knowing without additional context. However, in most cases, you can figure it out by looking at the verb tense.
Eg: वह कल या परसों गया - 'He went yesterday or the day before yesterday' because it is in the past tense
वह कल या परसों जाएगा - 'He will be going tomorrow or the day after tomorrow' because it is in the future tense
If there is some scope for ambiguity and you want to remove it, you can qualify it as गुज़रा हुआ कल/ बीता कल - 'yesterday' (literally 'the कल that is past') or आने वाला कल - 'tomorrow' (literally 'the कल that is coming') though these phrases are also used to talk about the past and the future in general.
Eg: मैं बीते कल के बारे में नहीं सोच रहा - 'I am not thinking about yesterday' (or 'I am not thinking about the past')