​
Get token balance

import { mkt } from './config'

const balance = await mkt.getTokenBalance('<COLLECTION_ADDRESS>', <TOKEN_ID>)

​
Check if you are the owner of the token

Internally calls getTokenBalance and returns true if balance is not 0n. 
import { mkt } from './config'

const isOwner = await mkt.isTokenOwner('<COLLECTION_ADDRESS>', <TOKEN_ID>)