You can identify the favourite from the underdog based on who has the higher odds to win. Favourites have a higher probability of success thus have higher odds. In betting odds, the favourites have a smaller payout and thus have smaller numbers.
In a win-or-lose scenario (no draws), favourites usually manifest like this:
Decimal: 1.80 or lower
American: -125 or lower
Fractional: 4/5 or lower
You will need to bet more to win less when it comes to favourites in a win-or-lose scenario while you win more for less while betting the underdog. It all correlates to their chances of success.