You can either use if statements to check if the health value is between two given min/max values, or you can use the Mathf.Clamp function to create a temporary value between a given range and check if it still matches your original value (in which case it is within range).
Mathf.Clamp solution:
public int HP=100;
public int HP_Max=100;
public int HP_Min=80;
// check if HP value is between 80% to 100%
if (Mathf.Clamp(HP, HP_Min, HP_Max) == HP)
{
// HP value is between 80% to 100%
// do something
}
And the alternative using if statements:
if (HP <= HP_Max && HP >= HP_Min)
{
//HP value is within range
}