You can check the cv joint by visual inspection and then by feeling. Visually inspect to see if either the inner or outer boot is torn and has slung cv grease around. Then if you grab the cv axle it should be pretty taught. If it has some lateral movement in and out (towards wheel towards engine) that is alright. However, if it has significant lateral movement and radial movement that might be a sign of a failing cv joint. As well, if there is any clunking metal on metal noise at the joints this could indicate a failing joint. The symptoms of a failing joint for me while driving were a clunking noise when turning and a significant vibration with throttle on around 65mph or more. Throttle on with a grade at 65mph plus the car had a significant front end shake.
For the motor mounts, I had no idea how shot they were until I had them replaced. At start up my car had a descent amount of movement from the engine shaking. As well, it idled rough. Now, I cant even tell the car is running at idle. When coming to a stop, it felt like the car would stop and then it would push forward again. This was the engine moving forward.

For the issue you are describing it probably wouldn't be related to motor mounts. A worn cv axle could be a good thing to look into though.