Is there a way to tell whether an act is innovation (bida')? For instance, offering namaz wahshat for a non specified marhum even out of its time between Maghrib and isha.

As far as Mustahab namaz is concerned, there is no restriction for its timing and other related issues as it is said for wajib namaz. Therefore it’s not an innovation if anyone with the Niyyat of reja offers salatul wahshat for a Momin who has passed away.