Sugata + measurements help to détermine the era but it's still a gamble . That said, there are many indications that a sword may have been made to resemble an older one. The thickness of the nakago, the size of the machi, the patina, the shine around the mekugi ana or on the nakago shinogi; the cleanliness of the inside of the "old" mekugi ana. The location of mekugi ana which is often fanciful. The number of mekugi ana is in no way a proof, nakago of the sword below is suriage and has only one mekugi ana whereas the Tomonari I posted on a previous post has several nakago ana and is ubu.
I've provided a drawing showing the position of the nakago shinogi on a nakago ubu and on a nakago suriage (I've exaggerated the positions of the shinogi of the nakago suriage so that it's easy to understand) which is explanatory enough.