We would need to know the circumstances to answer this question.
species and genus
What you want to know is not really clear, but if you are inquiring about the need of changing "say" to "tell" in indirect speech I must tell you there is not.
How can scientist tell if there are mutations
The noun clause 'that I love you' is the indirect object of the verb 'tell'.
It might be, but you can't tell without seeing how it is used in the sentence. Us is an object, but it can be either an indirect object or a direct object, depending on context. For example, in "Tell us a story", us is an indirect object. But in "They robbed us", us is a direct object.
scientist tell you that so they can inform you on things
Both "say" and "tell" can have a direct object and an indirect object. The direct object is the words spoken, and the indirect object is the person to whom the words are spoken. However, with "say" you put the word "to" in front of the indirect object; whereas, with "tell" you do not: I say to you, "Hello". I tell you, "Hello". Also, with "tell" you ALWAYS have an indirect object. With say, you don't have to have an indirect object: I say, "Hello". I tell you, "Hello". (You would never say, "I tell, Hello".)
by palpation
indirect characterization
when an author doesn't tell you a character's personality directly so you have to figure it out based on the character's actions.
Scientists obtain empirical evidence through experiments, where they manipulate variables and collect data, and through observations, where they gather data by directly observing phenomena in the natural world. Both methods are crucial for testing hypotheses and drawing conclusions based on evidence.
Scientists collect data about the amount of water vapour in the air because if there is a lot of water vapour, they can tell you that it might be humid, or that it is going to rain, or if there is barley any, you can breathe more lightly, HOPE THAT HELPS:)