A tooltip promises 120 damage over time, while another skill deals 80 immediately. The larger number does not automatically make the first skill better for the next fight. You need to know how much of that damage can actually happen.
Use an imaginary game example: a burn deals 20 damage at the end of each second for six seconds. If the target remains vulnerable for all six ticks, the total is 120. If it disappears after three ticks, only 60 has landed.
Choose the same window
Compare both options over the same period and under the same conditions. Include the time required to cast, travel or apply the effect. A direct hit that lands immediately and a delayed effect should not be compared as though their timing were identical.
Check whether refreshing an effect resets its timer, extends it or replaces remaining damage. Games handle these mechanics differently. A repeated application may add damage, waste remaining ticks or create a separate stack, depending on the actual rules.
Also note target immunity phases, movement and the possibility of changing targets. Damage that remains on one enemy may be less useful when the encounter suddenly requires attention elsewhere.
Test without changing every variable
In a safe practice setting supported by the game, keep equipment, target and buffs consistent. Record the game version and the number of attempts. Random critical hits or temporary bonuses can make a single result misleading.
Read the current skill description and patch information for the particular game before applying the example. The numbers here explain a comparison method; they are not statistics from a real title.
Finally, include resource cost and the rest of your action sequence. The useful question is not which tooltip has the biggest total, but which option delivers the needed effect within the encounter while leaving room for the other things your character must do.