More a process of elimination really/trial and error.
She's past teething, so we know it's not that.
We've dropped the bedtime milk and drinks during the night, so it's not that.
We don't let her into our bed for cuddles any more, so it's not that.
And it's not night terrors.
The days she naps, it takes until almost 9pm to get her to sleep.
The days she doesn't nap, she conks out by 8pm at the latest.
And as long as she has something fun to do, she is happy to skip her nap and doesn't conk out on the sofa. She gets ratty around tea time, but we get past it.
I can only suggest trying it - we did before Christmas and she wasn't ready. Her nursery advocated trying every other day with naps to see what was best for her.