It’s pretty easy to see generally by the level of quality that is delivered.
Just like telling if a car is a old Toyota VS a new Mercedes.
Most websites on the internet are poorly made due to the main reason that work was done overseas and secondly the budget provided usually was quite small meaning the developer & designer only could allocate a certain set amount of hours to do the work.
Overall though you should be able to tell by their quality of their website and their existing work. It’s important that you make sure that the work is done inhouse and not outsourced as you will find when things go wrong, it will cost you in time & most importantly in the quality of work that comes out. A real good WordPress designer should have passion for their work and will not roll out amateur looking work if they take pride in their work.