Network Load Balancing simply round robins between servers, and makes sure that any client goes to the same host (called affinity) so that the client always reaches the same host in case it has a session on it.
As such, any problems with SharePoint related to performance will not have anything to do with NLB, as all it does is decide which host a client will send requests to. Rather such problems would be down to one of the web front ends responding slowing – usually down to the application pool needing to free up memory, which should happen automatically after a while. Where NLB confuses things is that you can’t easily tell which WFE a client machine is talking to.
If NLB is not working, the only likely symptom would be that no requests to the NLB IP address would work. If you can ping your NLB IP address, then NLB is working.
You can check Network Load Balancing by going to Start > Administrative Tools and running Network Load Balancing (most likely you have this on your PC). You need to connect to one of the nodes involved. You can then see the status of the nodes. This should always be checked first in case one of the nodes has stopped responding on the NLB address). You should also use this interface to drainstop a node before taking it offline for any reason – that way you get as graceful as possible a handover of affinities.
Update: It seems that in certain circumstances, NLB can come apart under load. We recently switched from two high end Cisco core switches to two stacks of three Dell PowerConnect 6048 switches. Since then NLB works, but as soon as we get moderate load coming to the NLB address, Sharepoint grinds to a halt. Dell are chasing this, and we are promised a fix soon – until then no NLB – we can’t roll back to our Cisco switches. Christian Aid isn’t such a huge organisation (~800 staff worldwide), but heavy users of Sharepoint for file storage – seems enough to break NLB. I think it is something to do with the multicast hack Microsoft use to make NLB work – but I’d better wait to hear back from Dell and get it working before I can say for sure.