I don't think you can. Unless the data is massively large, surely just bruteforcing is just the way.

1) Read first time stamp
2) Calculate next 15 minute boundary time stamp
3) Search for it from there - if a later time stamp is found first, file is borked. Success if end of file is reached.
4) Calculate next time stamp boundary (add 15 minutes), goto 3

Alternately, filter out any markers that are not on the exact 15 minute boundary first and then do the same run through. Need to be careful in case there are non regular markers exactly on the 15 boundary since you won't filter them out (unless you can identify them and filter them out too). Then it's just a case of making sure each entry is 15 minutes after the previous one.