He works as a doctor, and people come to him to ask for sick notes, describing their symptoms, but they may be lying
Question

I work as a doctor, and a patient comes to me and tells me what is wrong with him, wanting to get sick leave, but I am not sure about what he is telling me, because there are some health problems that the doctor cannot say are imaginary. If I give him a sick note [so that he can get sick leave] in this case, is there any sin on me?
Praise be to Allah.If the sickness is obvious, then there is no problem with giving the patient a sick note [so that he can get sick leave], based on his illness.
If the sickness is not obvious – rather the doctor is relying on symptoms that the patient mentions to confirm that the patient is indeed sick – then the basic principle is that you should think positively and assume that a Muslim is telling the truth, and you should give him a sick note as appropriate to his illness, so long as you do not think it most likely that he is lying, such as if there is circumstantial evidence to that effect.
Shaykh al-Ameen ash-Shinqeeti (may Allah have mercy on him) said: Acting on the basis of what appears to be the case is essential, unless there is evidence to indicate otherwise.
End quote from Adwaa’ al-Bayaan (5/381).                 
If you give the patient a sick note based on what he says and he is lying, then there is no sin on you, because you judged the matter on the basis of what appeared to be the case, just as a ruling must be based on what witnesses say even if they are in fact lying.
The Prophet (blessings and peace of Allah be upon him) said: “I am only human. You may refer a dispute to me, and perhaps one of you is more eloquent in making his argument than the other, so I judge according to what I hear. If I judge in a person’s favour, giving him what is rightfully his brother’s, let him not take it, for I have only allocated to him a piece of fire.” Narrated by al-Bukhaari (6967) and Muslim (1713).
So what is required is to examine the matter, and not help the patient to get sick leave that he is not entitled to.
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) was asked: Is it permissible for a doctor to give someone a sick note, especially in the case of employees, when a person does not really need sick leave and the doctor did not examine the patient or examine him. Is the doctor sinning if he gives the patient more sick leave than he is entitled to?
He replied: In as-Saheehayn, it is narrated from Abu Bakr (may Allah be pleased with him) that the Prophet (blessings and peace of Allah be upon him) said: “Shall I not tell you of the gravest of major sins?” They said: Yes, O Messenger of Allah. He said: “Associating others with Allah, and defiantly disobeying parents” – and he was reclining but he sat up and said: “and false speech, and bearing false witness.”
Undoubtedly if the doctor give someone a sick note when he is not sick, there is no doubt that this is false speech and bearing false witness, and he is sinning, and he has committed one of the worst of major sins.
By the same token, the one who takes this sick leave is also sinning, as he is lying to his bosses and consuming wealth unlawfully, because the salary that he gets during this sick leave is something that he has taken unlawfully.
Similarly, if the doctor gives him more than he needs – such as if he needs three days sick leave, but the doctor gives him four – this is haraam and is one of the gravest of major sins.
End quote from al-Liqaa’ ash-Shahri (7/4).
And Allah knows best.