a model should wear a basic outfit unless the casting states differently.
Usually when I go to a casting this is what I wear:
A basic white or black tank top, a tube dress, denim shorts, or basic shorts, high heels and sometimes there is a pair of sandals in my bag.
Be aware of your outfit being too sexy.
If you are going to Maxim that is one thing, but if you are going to a shoe casting for a brand that is conservative then you should dress that way. Don't wear super short shorts where the curve of your booty shows.
It is also important to have nude thongs, sometimes for swimwear or lingerie castings you will have to try on lingerie and such and you will need to keep your thong on and the standard color is nude for your underwear.