While playing Magic: The Gathering with a friend, I noticed the deck he was playing with consistently had mana issues and didn't get a drop before turn three. To exaggerate the deck's issues, we played a variation of magic. "Opening Magic"
In Opening Magic, games are very short. Start like usual, up until you would decide on mulligans (iow: shuffle, cut, draw, think). Consider the hand you have, and if you would mulligan it or not. Does the hand have enough lands to get out of the opening game? Do you have drops in the first few turns?
Hidden from view of your opponent, set your life total to seven if you'd play the hand, or six if you would mulligan. Place the number so that you'd be moving the view obstructing object, not the dice/spinner/whatever-you-use-for-life-points. Give the hand face down to your opponent (who has been doing the same you have done with their hand).
Now consider their hand, and whether or not you would mulligan it. Once your both decided, let them know your choice on the second hand, and see if it matches theirs. After this, discuss any borderline mulligans and disagreements the two of you had. Feel free to grab the first couple of draws to try and drive a good mulligan point home.
Once this exchange is complete, graveyard any revealed cards and repeat the process until the libraries are empty.
This can be done in solo as well to learn what is going well with a deck, and what isn't. If you have improvements to this, or similar games, please comment away.