How much of my Social Security check is taxable?

Social Security benefits are subject to federal income taxes if your income is high enough. Currently individuals pay taxes on 50 percent of their benefits if their combined income is $25,000-34,000. Individuals pay taxes on 85 percent of their benefits if their income is over $34,000.

For joint return filers, the 50 percent tax level runs from $32,000-44,000, and the 85 percent level starts at $44,000.

"Combined income" = AGI + 'tax-free' interest + half your Social Security benefits

See here for details :
http://www.ssa.gov/planners/taxes.htm