How can I know if these two signal are orthogonal to each other So the question is to determine if this statement is true or false. The question can be seen below. 

I know that two signals are orthogonal if their inner product is equal to 0. I know how this calculation is done with vectors but not with signals. Would be really helpful i someone could explain this to me. Is it also possible to just directly see it in the two graphs if they 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.