In Laravel it is very normal to get data from related objects like $customer->wallet->balance.
What is the best approach in testing the value of the customers' current balance?
Sometimes this even goes a layer deeper.
There's no "best approach" but in my opinion you'd be better off writing two test classes, one for your Customer model and one for your Wallet model. Then in each you'd confirm that they can link to each other through a belongsTo or hasOne relationship.