Not sure why you would put coffee on your face. People use coffee grounds to reduce cellulite. It is a temporary solution. You could use coffee grounds on your face to exfoliate. Be careful. Coffee grounds may be too harsh for your face. The dip is largely due to two years of surplus from Brazil, the world’s largest coffee producer, which has had a serious impact on growers around the world by pushing millions of kilograms of beans onto the market. Economic issues in coffee-producing regions like Central America and Africa are also at work. Egg white works magically on open pores and oily skin. It also dries out pimples and tiny bumps. It also helps with blackheads to some extent. But, if you have dry skin, then egg white will extremely dry out your skin. Also, I don’t recommend this if you have sensitive skin. Unless something has an adverse effect, it would not normally be regarded as being toxic. Long term exposure to materials that accumulate in the body (e.g. mercury) would result in toxicity especially when they reach critical levels. The methods to detect accumulated toxins would depend on the material involved but, for arsenic for example, could be as simple as chemical analysis of hair or fingernails. Chemical analysis can also be performed on material from dead people if poisoning is suspected, e.g. on the bile from the gall bladder of an exhumed body using mass spectroscopy.
Some materials (e.g. caffeine or smoke) can have cumulative and long-term effects on DNA without themselves necessarily accumulating in the body. One very sensitive method for detection of this type of damage involves looking for altered DNA using the p32 post-labelling technique. Alterations to DNA can lead to mutations that, in turn, can have long-term effects including aging of the skin, genetic deformities and increased rates of abortion in the offspring, much of which is difficult or impossible to evaluate in humans (e.g. by epidemiological studies) which is one reason why cultured cells, rats and other animals are still required to be used to evaluate potential effects before new materials can be marketed. Materials used in cosmetics (including face cream, shampoo and toothpaste) have previously been tested for toxicity in animal etc models so (at the levels present) are extremely unlikely to have any adverse effect beyond perhaps skin sensitivity in a few hypersensitive individuals. Yes, he liked the coffee and wanted another cup. Most people have a coffee shop they frequent daily. Also, many coffee shop employees put little symbols (happy faces are common) on the cups. Unless he asked you out, or went out of his way to have you serve him instead of someone else, I wouldn’t read too much into it.