There are so many things to look for! A comprehensive list is not available. Here are some general comments:
1. You need to learn how to recognize a silver coin from the look of it.
2. Look for current redeemable foreign exchange (larger denominations) of the major economies-like Euros, British pounds, Swiss francs, Canadian....
but even with this you need a local source who will buy them for 50% of the rate for paper.
3. Look for anything pre-WWII in XF+ condition, or pre-wwI in VF+, or anything pre-1800 that is at least VG. check these against the values in a world coin catalogue.
4. Sort them all into countries, and the countries into type. You will soon realize what the common/ modern stuff is, 99% of which has minimal value.
5. large coins (UK penny and bigger) are usually of more interest and are more sellable.