It'll feel like shit, sound awful and stink... So the same as any other diesel. 
But seriously, with the EGR open and the intercooler bypassed you really do feel a lack of acceleration, mine used to jerk badly during a regen as well. You also get a distinctive noise due to the later injections and a hot cat smell, possibly an extra 'dieselly' smell as well. The fan running when you haven't got AC on and having been thrashing it or stuck in traffic is another good indication that a regen is taking place. I could also tell by the MPG dropping off when it should be rising (on the flat at 30mph for example) but for those less anal about mpg that might not be so easy to spot lol.
But seriously, with the EGR open and the intercooler bypassed you really do feel a lack of acceleration, mine used to jerk badly during a regen as well. You also get a distinctive noise due to the later injections and a hot cat smell, possibly an extra 'dieselly' smell as well. The fan running when you haven't got AC on and having been thrashing it or stuck in traffic is another good indication that a regen is taking place. I could also tell by the MPG dropping off when it should be rising (on the flat at 30mph for example) but for those less anal about mpg that might not be so easy to spot lol.