If you're buying it legally it'll usually be on the package. For instance I have a packet here on my desk that says "Blue Dawg - j32 80% Indica / 20% Sativa". There are also websites that can tell you about different strains - leafly.com, allbud.com, and nuggetry.com, to name a few that come up when I search for 'blue dawg marijuana'.

Mood and delivery method can alter the effects, too. If you're taking massive bong hits after a long day at work it'll be a lot easier to fall asleep than if you're hitting a vaporizer early in the day.