Within a DAW, as far as i know, delays of up to a few frames can be caused by processor-heavy plug ins. I'm not sure if Nuendo has a delay compensation feature, or a way to measure the delay in your chain, but maybe someone else can help there.
Different monitor outputs can bring delays with them too, although if you're plugging the monitor straight into your computer (as opposed to using a Canopus box or something, which converts the signal to component), the delay shouldn't be huge. Play a pip (without any plug ins in the chain), accompanied by a flash, and watch. Does it seem out of sync to you? One trick i use, in lieu of a sync checking device, is to do this and watch the flash in my peripheral vision. Just relax; don't be in sync-checking mode, just be aware. If you focus too hard on sync, your brain can artificially impose it. If you do have a big delay in your monitor, this will make you cut your FX in earlier than necessary, and you will need to offset your video (again, i'm not sure if Nuendo has this feature).
Finally, what kind of sync problems are they reporting? Does it slowly drift out of sync, a few frames every few minutes? More? Lately, i've had some clients report sync drift issues, although the majority have been totally fine. I haven't figured out why yet, but i think it might be the picture editing software assigning a frame rate to my audio and adjusting the speed while importing. Ask for a pip at the head and tail of the program in future. This way, they can tell you whether they line up, and if not, how many frames you're out by.
If sync starts out ok, and drifts out by about 18 frames every 10 minutes, then you can try pulling your audio up/down by .01%.
If you're a few frames late from the start, then it's a processing delay in your system. Although if they line up your front pip with theirs, it should be in sync.
That's all i can think of pre-coffee. Hope it helps.