Guaranteeing that a sniffer only sniffs, not sneezes

We need to do some testing on a product that consists of a networked system of computers. We'd like to do some verification testing where we sniff the traffic on the network to demonstrate that things are as expected. One thing we're thoughtful of is that we only want to observe, and not add any traffic.

1. Is it correct that Wireshark and similar applications act as we need?
2. Is there some hardware that we can add to ensure that packets only go into the sniffer, and it doesn't inject anything of its own?

Thanks in advance.