I've learned to do the tug test on any and all threads before using them.
I usually can't tell by looking if thread is still strong or not.
I've used Coats and Clark cotton covered polyester Dual Duty thread for years (why did they quit making it?) and I have quilts that are over 20 years old that are our couch quilts - the bindings are frayed - but no fabric has been "cut" by the thread that was used.