The first thing to do it so look at passive and active ranges of movement in relation to their pain. Then to also look at the effect of isometric movement in different ranges. Generally speaking pain during passive and active range might well point towards some type of joint issue, be that arthritic, capsular, ligamentous etc. Pain on active movement yet pain free on passive might point the ball more to muscular involvemet. This of course might be combined with joint compression so you can rule it out and should look for patterns of pain at specific points in the range of motion or throghout the movement. The last thing is to look at isometric issues in different ranges of flex/ext. See what matches up. If pain is joint range specific on isometric testing and also on active movements it might still be joint and mm together. If it is painful thoughout active and isometric ranges but pain free on passive testing the perhaps it is more likely to be muscular or tendinous.
The main thing is to do the testing as best you can. You should always test above and below the symptomatic joint, i.e. for the hip this would be the pelvis, lumbar spine and knee joints.
Although your question does ask more about the tendin vs. the muscle remember that a tendon is the end/attachment of a muscle. And a tendon issue can only be brought about by a stress to the muscle first. Where it is might tell you what is stressed/strained but all biomechanical considerations should be considered in deciding why?