There are a few different ways to mention this, at various stages of your job search. Don't hesitate to use all of them if the opportunity arises:
If the application form has a space for "reason for leaving" (many do, including online applications), indicate "laid off" or if that is not an option use "other" and indicate "laid off".
In your cover letter or introductory email, say up front that you are currently looking for a new position because of your previous company's reduction in force.
In a phone screen or other interview, when they ask why you are looking for another position, indicate your company has gone through a reduction in force and you are now laid off.
If you are indicating in the application (where possible) and in your introductory email or cover letter that you have been laid off, then you've done what you can to get the point across. If people aren't reading those, and are just looking at your resume, then that's unfortunate but not uncommon.
You could mitigate that by indicating your reason for leaving directly in your resume in this situation in the entry for this position, but I don't see that very often. If, as a hiring manager, I did see a statement about the RIF directly on your resume, I would consider it valid and wouldn't think it was too weird (but then again I would also expect to see it in your cover letter).