You can use an infrared heat detecting gun to troubleshoot a hydraulic system.
Our website contains dozens of articles about troubleshooting hydraulic systems. In fact, I recently finished posting a series of articles called, "Think First, Then Troubleshoot!" This series explains how troubleshooting requires a systematic approach to identify and correct the source of malfunctions, not just treat symptoms.
Troubleshooting requires tools to assess the condition of a hydraulic system by reading pressure, flow, and temperature. Monitoring temperature can help you gain an overall idea of what's going on in a system, but an infrared heat detector can provide specific information — information than can pinpoint the source of problems.
To learn more, read the article I just posted by clicking here. The article was orignially published several years ago, but the techniques described in it are just as applicable today.