Checking if an associated object has failed to save/create
Given the current setup, the simplest thing would probably be to raise an exception using #save!:

def apply_to(listing)
  jobapp = Jobapp.new(resume: self, listing: listing)
  jobapp.save! # note the bang (!)
end


And in the controller

def apply
  set_resume
  @resume.apply_to(@listing)
  redirect_to @listing
rescue ActiveRecordError
  redirect_to @listing, alert: 'Unable to apply for job'
end


It may be worth the effort to rename your method to apply_to! (also with a bang) to make it clear that it can raise an exception.



As for why create doesn't work, it's not that it returns true - it returns the new record (and all objects except nil and false are considered "true"). To check if create actually managed to persist that record in the database, you can check record.persisted?. If true, then it was saved just fine.

So using your create version of the method, you could do

def apply
  set_resume
  jobapp = @resume.apply_to(@listing)
  if jobapp.persisted?
    redirect_to @listing
  else
    redirect_to @listing, alert: 'Unable to apply for job'
  end
end


This would also let you access jobapp.errors, if you want.



A few other things caught my eye:


set_resume should probably be in a before-filter. Or be a method that just returns a record, leaving it up to the caller to stick it in an instance variable (or not)
I imagine a jobapp belongs to a listing (and/or a resume). If so, it'd be cleaner to build it like so:

jobapp = owner.jobapps.new(other: foo)



Given that it's this simple, though, it might be something you could just do in the controller.

It's never quite clear where a join model like jobapp (which I assume is basically a join model) should be created. Should it be in the Resume model or in the Listing model? Either one makes sense. So the Solomonic solution is do neither (at least for now), and handle it in the controller:

def apply
  jobapp = @listing.jobapps.new(resume: @resume)
  if jobapp.save
    redirect_to @listing
  else
    redirect_to @listing, alert: 'Unable to apply for job'
  end
end


Lastly, you might want to tweak that error message. It seems a little "blunt" to me. Though it's the server that failed, it sounds as if the user is unable to apply for the job. "Failed to save job application" might be more appropriate