Depends how you are ordering the PCBs. If you order the single PCB, then use "Panel by JLCPCB," the stencil is designed by the person generating your panel. You'll have to make sure your paste mask is properly constrained prior to ordering. If you generate the gerbers yourself already in a panel, you can manually view the gerbers in your preferred viewer. The stencil will be designed off the paste mask layer. One thing to keep in mind is that whenever you order a stencil the orientation is not specified unless you specify exclusively in the "Stencil Remark" field. Been bitten by this a couple times, so now I specify each time.

The PCB isn't panellised.  Just need the stencil for applying the paste by hand.

@MikeDB Then you're only option is reviewing the paste mask layer of your gerbers prior to ordering.

Yes I thought that might be the case.  Worrying time waiting for it to arrive.

@mikedb - they've been good for me so far :-) In my case, they dispense a little too much paste but not unmanageable. Curious to hear about your experience.