I want a preference to prevent me from editing read-only files

I use an SCM (Perforce) that keeps most of the source tree read-only; sometimes I forget if a file is checked out for editing or not, and start editing a non-checked-out file, only to realize it when I try to save. I'd like to instead be able to have ST2 just yell at me if I attempt to change a read-only file. Lots of other editors have this setting, please let me have it in ST2 as well. (If there is such a setting, well, nevermind, but I haven't been able to find it so far...)