A diploid that has been treated to try to convert it to tet is usually a chimera. The treatment rarely (although sometimes) manages to treat all the necessary parts to make it a complete tetraploid.
If you take a diploid cultivar, say Siloam June Bug (SJB) and treat it to convert it to tetraploid and the conversion works then the tetraploid SJB will have pollen that is about 30% larger in length and width than the diploid (SJB) version's pollen. That is because doubling the number of chromosomes tends to double the volume of the plant's cells and a cell or pollen has about double the volume when its length, width and height are all increased by about 30%.
But different diploid cultivars have different size pollen and different tetraploid cultivars have different size pollen and triploids have a wide range of pollen sizes that falls in between. It is quite likely that there are diploid cultivars with pollen that is larger than some tetraploid cultivars (and/or vice versa).
I don't think anyone has examined diploid pollen from a range of cultivars, small flowered, large flowered, spideries, etc., nor has that been done for a range of tetraploid cultivars.
One can also look at the size of guard cells in the leaves but the same qualifier holds for them as for pollen - some diploid cultivars may have guard cells larger than some tetraploid cultivars.
Bottom line, although pollen size examined under a microscope and measured can give a reasonable idea of whether a plant is a diploid or a tetraploid it is not necessarily a certainty. The only certainty is to have a chromosome count done of the root tips (but that does not necessarily work for attempted diploid to tetraploid conversions - which would require chromosome counts of the pollen).
There are some very rare conditions when a true diploid (pollen) crossed on a tetraploid (pod) may produce a viable seed. The seedling could be a triploid or very rarely a tetraploid. The opposite cross of a diploid pod with tetraploid pollen may also very rarely work to produce a viable triploid seed. Those rare conditions would typically require hundreds of pollinations to get any seeds in some specific crosses and would always fail in other specific crosses.
There are also a few fertile triploid cultivars.
