They do contain hydraulic fluid- when I had a front/side shunt a couple of years ago one failed (right-side looking from front) and the fluid leaked out. Had really bad vibration at idle/very low speed 'till I changed it. Probably obvious, but did you make sure the small tab in the top of the mount was aligned with the notch in the engine bracket?
I don't remember the new one making a sloshy fluid sound, but I also may not have shook it to check
You don't need to remove the crossover pipe to change them out btw, or at least I didn't...
As Sixto said you should be able to get a finger in the gap between the frame and the bottom of the mount, it'll be obvious if it's failed.