Only one sure way that I know of.

Sample the goods.

Be prepared for her to tell that she's plenty tight but that you're just too small.

There are fat cells and layers in every body part, the vagina too, BBW are supposed to be tighter. I'm a provider and never been with a BBW but some clients told me about this.

Also dependent on whether or not she practices her kegels on a daily basis. A provider can have kids, but if she's been doing her kegels throughout it, you'll never be able to even tell she even had kids.... unless she tells you. But, as someone else pointed out, you wouldn't know this until you "tried out the merchandise".