If you brewed from a kit the recipe will tell you what the OG should be for a 5 gallon batch, and this figure is pretty reliable.
Otherwise you will have to figure it out based on the malts you used, the amounts, and the final volume you had.
Typically, people take a gravity reading right before they pitch their yeast to find out, though this can be misleading when brewing a batch where most of the fermentables are from extracts as their viscosity can make it difficult to mix them in thoroughly before you pitch. Meaning your sample can be higher or lower than what it actually is due to samlping a more or less concentrated area of wort.