"Ewan ko" is a common Filipino expression. It used to be also "aywan ko", but it seems that it's now always written as "ewan ko".
When saying "ewan ko" in a matter-of-factly way to strangers, it may sound dismissive. So, it may be safer to use the literal translation of "I do not know", which is "hindi ko alam", as it would sound more polite. If, for example, a stranger asks you if you happen to know where he can buy beer around that area you're at, replying with "ewan ko" may come across as something like, "I don't know and I don't care". So, "hindi ko alam" would sound more factual and honest.
In general also, if a question is stated as "Alam mo ba?" (Would you know?), the more appropriate negative response would "Hindi ko alam..." because it would conform to the wordings of the question, plus, it would also sound polite.
John: "Sino 'yung kasama ni Tom?" = Who is that with Tom? Mary: "Ewan ko"/"Hindi ko alam" = I don't know. (either response will be OK, although at times we even say both for emphasis)
John: "Alam mo ba kung sino 'yung kasama ni Tom?" = Would you know who's that with Tom? Mary 1: "Ewan ko" (would be a curt response, but may be all right among friends) Mary 2: "Hindi ko alam" (would be the more natural or straightforward response to the question)
"Ewan ko kung/Hindi ko alam kung dumating na si James" = I don't know if James has arrived.
"OK iyon sa akin, pero ewan ko/hindi ko alam sa iyo kung OK iyon" = It's/That's OK with me, but I don't know with you if it's/that's OK.
"Ewan ko lang/Hindi ko lang alam kung papayag si John" = I just don't know if John would agree (Please note that "lang" (just) follows "ko" in both cases)
"Ewan ko/Hindi ko alam sa iyo" = I don't know with you.
"Ewan ko/Hindi ko alam (kung) alin ang sa iyo" = I don't know (as to) which one is yours. ("kung" translates to "as to" in this case)