Mine positions are chosen at the start of every round and can be verified after the round ends. This is accomplished by using 2 strings:
When a game starts, you see the SHA-256 hash string to ensure that the secret string will not change when you make a bet. When you make a bet, the secret string is revealed along with the bet result. At this point you can check that the hash string is corresponding to the revealed secret using one of the third-party websites like:
The structure of the secret is as follows: <mine_pos_1>[-<mine_pos_2>[...[-<mine_pos_N>]]]__<salt>.
For instance, a secret 10-13-25__11f5544e3d1c0a6a means that mines are located in positions 10, 13 and 25. Mine positions start from 1 in the top left corner of the field and then enumerated from left to right, top to bottom.