I am a resident of California but earned $1,850 in Illinois as a 1099 Misc. Do I have to file an additional state return as a non resident? How do I figure out if this is under the exemption or not?

As a non-resident, you have to file a non-resident IL tax return if the amount of IL-source income exceeds your non-resident exempton. To figure your exemption you use form IL-1040 and schedule NR, but it boils down to this:

1. Your base IL exemption is $2000 times the number of exemptions from your federal form 1040, plus an additional $2K if you are 65, plus another $2K if you are bind.
2. Multiply the total of your base exemption by the percentage of your income that is IL source. For example: if your total income is $100K, and your IL portion is $1850, then the pecentage of your income that is IL source is 1.85%. If you have 2 exemptions (married filing jointly, no kids as an example) then you have $4K times 1.85% = $74. Hence your IL source income exceeds your IL exemption, and you must file a non-resident return with IL.

You can get the instructions and forms here:
2010 Individual Income Tax Forms (http://www.revenue.state.il.us/taxforms/IncmCurrentYear/Individual/index.htm)