Brief Answer:
Physical examination / Investigations to get a diagnosis.

Detailed Answer:
Hi.
Thanks for your query.

Noted your history of falling sick on regular basis, weakness in joints, smoke and drink regularly and want to know if this is due to stomach flu or liver dysfunctionality.

From your history, have you not yet visited a Doctor ?
Have you tried any medicines?
Done any investigations?
Any other symptoms you have like fever off and on, any respiratory or abdominal problems? Headache ?
Please give more information for me to help you more.

I would suggest in a patient with similar history the following:
-Clinical / physical examination by a MD Medicines Physician. This is very important as actual examination of the body gives lots of information like- Temperature, pulse, Blood pressure, lymph nodes status. liver and spleen palpation, condition of the heat and lungs and so on.
-Investigations:
Blood- CBC, ESR, Blood sugar, liver, kidney and thyroid function tests, Widal and Malaria by slide and antigen.
Urine- Routine, microscopy, culture and sensitivity.
-X-ray Chest
-Ultrasonography of the abdomen.
-Any other tests as per the clinical findings of the Physician.

This is a standard format in a case with a history as you have.
Once the clinical and investigations diagnosis are correlated, a proper treatment can be planned to give you cure.

Needless to say you have to reduce your smoking and drinking.

Now: your query: (want to know if this is due to stomach flu or some liver disfunctionality).
Stomach flu may not cause such problem on a regular basis unless you have other medical problems.
Liver dysfunction will be obvious by apparent jaundice, yellow urine and confirmed by blood tests.

I hope this answer help for an insight into a problem, proper diagnosis and curative treatment.

Please feel free to ask for any clarifications if you need to.

Wishing you early diagnosis and cure.

---