Short of taking a blood sample from the insect in question, I don’t think their is an easy way of finding out – they move around too quickly and are too small for a quick visual examination (assuming that something could be seen that would tell you if they were carrying an illness). Unfortunately I think that the only easy way to tell is if the population of a larger animal – such as humans – start to get the illness.
As Marcus has said, there’s no easy way to tell just from looking at an insect (at least that I know of). But, if you were able to get a sample of its “blood,” (insects have a different sort of blood than humans), then you could probably DNA-sequence what’s inside and figure out which sort of bacteria or viruses are living there.
This is very similar to a cool new field called “metagenomics,” which just takes a sample from the environment and tried to use DNA sequencing and some math to try to figure out which organisms are contained inside.
Unfortunately it’s pretty tricky to tell – as Marcus said they are too small and quick to examine, and I’m not sure how much use a visual inspection would be. As Alex said, this is beyond my area of expertise, so short of the suggestions the other engineers have made, I can’t add anything else I’m afraid.