One of the best ways to know if an engine is any good is by checking how the spark plugs look after using it for approximately 5 / 10km. We’ve compiled some spark plug guides here.
Here’s a simple guide that has to do with carburetor settings (c/o tuningmatters.com):
Here’s a little more comprehensive one, indicating issues that may have to do with oil seeping into the cylinder (c/o totallyamaha.com).
Here’s another good guide (c/o honta-tech.com). The first four images show relatively simple issues that may be fixed by a good tune up, but the 2nd to 3rd row can indicate far more serious issues that may translate to big $$$ in repair costs.
Conclusion: The important thing is to memorize the Optimized / Normal appearance. Once the spark plug looks anything other than Normal then there’s bound to be trouble, and you can use these guides to figure out what it is exactly.
It’s also important to note that while plugs are a good way of determining engine health, they cannot be relied on to relay the exact issue. You need to combine plug readings with such things as how the used oil looks, the quality of exhaust, and the general appearance and condition of the engine and the rest of the car to get a complete picture.