How can one tell the difference between implantation bleeding and a 1st-trimester miscarriage?

Indistinguishable. Going forward, these two possibilities are indistinguishable. Follow-up with serial pregnancy tests, or beta HCG determination in your blood will be the only way to tell if you are carrying a growing pregnancy. Take Folic Acid 1-5 mg daily - best of luck.
Bleeding. Implantation bleeding occurs before the missed period. Typically an earlier and lighter period. A miscarriage happens after the missed period and can be more painful.