Replacing engine mounts

Hello,
As I'm soon to be fitting a new stainless system I've been advised to check and possibly replace the engine mounts. I imagine this is to ensure there's not too much play during startup and acceleration which could stress the joints. Can this be done with the engine and gearbox in situ with, for example a jack gently taking the weight? 

Any advice gratefully received.

Thanks

Tend to find its the rear one that needs replacing.
DO NOT I REPEAT DO NOT, replace it for a poly one unless it's a track car.

When I fitted my 4 branch I found the down pipe would tap the anti roll bar when pulling away if the revs dropped a bit too low.
Changing the original 34yr old mount for a standard new rubber one corrected this.

Easy enough to do, get the front end up, crack the bolts, jack under the gearbox for support and then remove mount

WATERNAIR said

Tend to find its the rear one that needs replacing.
DO NOT I REPEAT DO NOT, replace it for a poly one unless it's a track car.

When I fitted my 4 branch I found the down pipe would tap the anti roll bar when pulling away if the revs dropped a bit too low.
Changing the original 34yr old mount for a standard new rubber one corrected this.

Easy enough to do, get the front end up, crack the bolts, jack under the gearbox for support and then remove mount

 Thanks a million for this, really helpful.