The easiest way is to check the well-known id.
if (sr.Wkid == SpatialReferences.WGS84.Wkid)
IsWGS84 = true;
or you could use the hard-coded wkid = 4326.
if (sr.Wkid == 4326) ... but occasionally these wkids are changed by EPSG so the first way is safer.