If you can do a compression test, you can verify if the compression is still within specs. A slight loose compression is not physically observable (like hatak ng takbo). Or if one or 2 valves lang ang may slight problem ganun din - hindi obvious. A compression test will show numerical figures sa tester and each engine will have its own specs of compression range.

My nissan sunny pick-up had this (inistock ko for 1.5 years kasi ng hindi inistart) so ayun the valve seat ng mga bukas ng valve corroded and thus the loose compression. One symptom I observed was hindi ko sya matiming ng ayos and the engine will die on low rpm (dapat mataas). I guess my case was extreme than yours kaya obvious and loose compression without the tester. There might me a need for some machine shop works. Total damage cost was P2950 sa akin....top overhaul job was P1200, gasket was P450, machine shop works was P880, the balance were spent for the valve seats and the gasket sealer etc.

Goodluck.