Welcome to the community. First the Normal distribution (Gaussian) is theoretical. No data will ever be perfectly normal. That being said there are number of indicators the distribution of your data is a close approximation of normal:
1. Visual look at the histogram....Bell shaped
2. Normal quantile plot...the dots should fall close to the straight line
3. Quantitative statistics...continuous fit>fit normal... Option>goodness of fit (Shapiro-Wilk, Anderson-Darling)
"All models are wrong, some are useful" G.E.P. Box
