So I think this is a more conversational kind of question. I will address some misconceptions you have, and I will try to keep it brief, considering the nature and depth of your question. One could comment on the question very deeply, so I'll stick to addressing some misconceptions.
Can I tell whether my sandwich is contaminated for example?
Usually you can taste contamination because of the (action of) metabolic products of microorganisms. You do not taste nor smell the microorganism directly; you taste their metabolites or the effect of their presence, such as products of fermentation, or oxidized molecules which are normally not present because air does not penetrate through e.g. apple skins. Take filamentous fungi, for instance; they release enzymes that act on complex carbohydrates to break them down. You cannot taste the cellulose which makes up the majority of a stalk of celery, but you may be able to taste its broken down products once a fungus has begun to digest it prior to absorbing its nutrients: sweet glucose, sucrose, trioses, some acids. It depends on the food and the extent and kind of contamination.
they are too small to have any taste and, even if you manage to eat a whole lot of viruses it would taste like water.
You can taste molecules which are far smaller than a virus. Indeed, pheromones and volatile or sticky chemicals and all that jazz are definitely much smaller than viruses. Viral particles themselves may actually be too big to taste, because your sense of gustation and olfaction depends on the interaction between tastant or odorant molecules and corresponding receptors, which are situated on the membrane of sensory neurons (nerve cells which are responsible for detecting and relaying information to the brain about the outside world). These receptors harbor ligand-binding pockets that are too small to capture entire viruses. This does not rule out the ability for you to taste broken bits of viruses, but I think this is unlikely or coincidental (i.e. the particle may bind to receptors weakly and not very well).
it would taste like water
Water is a requisite for taste and smell, things have to be dissolved in water for you to be able to detect them. Water by itself has no taste! Only impurities, oxygenation and carbonation and mineral content give water flavor. I understand that you factor in temperature and texture into the 'perception of water', but these are not gustatory phenomena and use different mechanisms of detection.