The program WebbPSF models Point Spread Functions for the James Webb Space Telescope, using predicted wavefronts based on the optical requirements and error budgets for the telescope and instruments. WebbPSF can produce monochromatic and broadband PSFs for all instruments in direct imaging and coronagraphic modes; spectroscopic modes are not yet supported.
A number of science simulations have been performed to investigate feasibility of observations and to guide optimal telescope and instrument design.
If you would like to know whether your observations are going to be feasible with JWST, you should look at the JWST Exposure Time Calculators or the Sensitivity page