A linear function creates a straight line when graphed on a coordinate plane. It is made up of terms separated by a plus or minus sign. To determine if an equation is a linear function without graphing, you will need to check to see if your function has the characteristics of a linear function. Linear functions are first-degree polynomials.
Check that the y, or independent variable, is by itself on one side of the equation. If it is not, rearrange the equation so that it is. For example, given the equation 5y + 6x = 7, move the 6x term to the other side of the equation by subtracting it from both sides. This yields 5y = 7 - 6x. Then divide both side by 5 so you have y = 7/5 - (6/5)x.
Determine whether the equation is a polynomial or not. For an equation to be a polynomial, the power of the independent or "x" variable of each term must be a whole number. The terms can be made up of constants and variables. If the equation is not a polynomial, it is not a linear equation. In the example, y = 7/5 - (6/5)x has one "x" term and its power is 1. Because 1 is a whole number, y = 7/5 - (6/5)x is a polynomial.
Determine whether the equation is a first-degree polynomial. Locate the exponent with the highest degree out of the terms. That exponent is the degree of the polynomial. If it is one, it is a linear equation. Because the highest power of "x" in y = 7/5 - (6/5)x is 1, it is a linear function.
TL;DR (Too Long; Didn't Read)
Make sure no variable is multiplied by another variable in the function. If that's the case, it's not a linear equation.