One way to check that is to use ice rods or wands, attack the boss until you get a decent amount of samples, analyse the data and compare minimum and maximum values with the actual minimum and maximum the weapon you're using.
For example, let's say I'm using a naga rod, a rod that deals ice damage from 90 to 110. If I attack a boss and check the damage values and my minimum was 45 and maximum was 55, I can conclude that the boss is 50% resistant to ice.
This has been the method used by the TibiaWiki team to determine the values, however since the Winter Update 2022 release it's impossible to reliably determine these numbers as all creatures and bosses in the game now have mitigation, which directly decrease damage received from all elements.