Start with evidence, not a guess
Photograph both faces and the edge in even light. Record every readable inscription, the visible date and date system, portrait direction, major reverse device, denomination, and mint mark. Do not clean the coin to reveal details.
Measure the physical coin
Use a scale accurate to at least 0.1 gram and a caliper for diameter and thickness. Note whether the edge is plain, reeded, lettered, or decorated. A safe magnet test can help separate some alloys, but magnetism alone never proves authenticity.
Narrow country and era
Match script, ruler title, national emblem, calendar, and currency unit. Remember that the issuing authority may be a former state, colony, occupation, province, or empire rather than the modern country where the coin was found.
Confirm with multiple characteristics
A reliable identification agrees on design, date, denomination, metal, weight, diameter, edge, and mint placement. If one characteristic conflicts, keep the attribution provisional and consult a specialist reference.