How do you know? When your dc goes into his/her room and closes the door on you and says "no, get out, my room".
Honestly that's what my dd did at about 22 mo.
Background - we always coslept. When we moved into a new house, we put a toddler bed in the 2nd br that my MIL had brought for dd. I told dd it was hers and this would be her room but I didn't really expect her to sleep in it. I personally find cosleeping easier. Well she took to it. The 2 yo MINE MINE MINE attitude I guess.
However, this room is across the hallway from our br and I leave a nightlight on so she can come into our bed at anytime during the night. She usually does this about 2-3 am. I just find her in our bed in the morning.
Or lately, she's nearly 2.5 now, the novelty has worn off a bit and she's wanting to start off sleeping in our bed, sometimes she'll stay there all night but sometimes she'll get annoyed at the lack of space (we have a Queen) and just announce she's going to sleep in her bed. On bad nights, she goes back and forth several times before she'll finally go to sleep.
I think all kids are different, just introduce the bed and follow your child's lead. Just beware toddler beds with gaps in the railings - dd would turn sideways and get a leg caught in one and wake up crying because she was stuck. In fact I think CPS are doing a big recall of Graco toddler beds at present. We also have a futon mattress on the floor (no base. Iit was for us to sleep on beside the toddler bed if dd was having a bad night, however dd now chooses to sleep there sometimes too.