How can I know if these two signal are orthogonal to each other
The inner product of two continuous-time signals is defined by

$$\langle\mathbf{s}_1,\mathbf{s}_2\rangle=\int_{-\infty}^{\infty}s_1(t)s^*_2(t)dt\tag{1}$$

where $*$ denotes complex conjugation. In your example the conjugation is irrelevant because both signals are real-valued. So you just need to multiply the signals and determine the area of the result.

With $(1)$ it should be easy for you to show that the two given signals are indeed orthogonal.