signs of a denial of service attack?

Recently a website of ours has been having some problems.  During specific period of the day the site slows to a crawl.  We believe it's a problem with the iis set up on the server.  However the company that we paid to develop this site and the apps it uses are insisting  that it's a DoS attack.  The server on that is hosting this site is also hosting about 8 others.  None of those sites seem to be having a problem, they are all just as fast and responsive as always.  The server doesn't seem to be getting overly taxed.  It's slightly above the recorded benchmarks we have for it.  We haven't noticed any drop in Internet performance, we are on a 100meg up down connection.  My boss wants to let them access our sonicwall and poke around in there to look for signs of a DoS attack.  I strongly objected to this, and said I would let them in with read only access.  My boss and the VAR objected and I was over ruled.  They say that since I've never been anywhere that has had this happen I don't know what I'm talking about.  So I was wondering, before I officially go to my boss and call them liars, if I was missing something.  I'll admit, I could absolutely be wrong, but my guts are telling me that I'm not.