Definitely generic medicines contains the same active ingredients as in Branded drugs so there is same effect with a lower price however the quality could be ensured by the manufacturing unit  generally medical companies do not compromise in quality and standards.  

The active ingredient(s) in both generic and branded medicines are the same and hence the same pharmacological effect is expected ideally. But quality, other ingredients(Non-active ingredients), the technology, expertise variation between the manufacturer's might bring some differences in the intended therapeutic outcome expected from the drugs. The quality of the drugs whether generic or brand is usually controlled by the manufacturer's themselves( drug companies), regulatory agencies(national/international), the Purchaser etc. Many quality parameters are checked starting from identity, quantity etc to bio-equivalency.

Generic medicine may be efficacious but potency may be the question.. For an example if brand A give result in 3 dose, then may be the generic A medicines may give result in 6 or more dose....

moreover there is history that generic drug may not contain active pharmaceutical ingredient as it's labelled...

As per my knowledge (theoretical and case studies), a generic medicine is approved by US FDA only if a comparison is made between the efficacy and potency of the medicine with the Branded one. However, spurious drugs are one example of generic drugs which fails to control quality. The quality check at a personal level is complicated. Government of India launched a helpline for the same named as Pharmasecure but unfortunately it did not work properly. For developing countries, consumer has very little power to check the quality. The reasons may be various. Price control, regulations, Investment etc. are the hurdles.

For now, I can only say that educated persons may use internet to check the quality but it is not possible for bottom 80%.

Generic and Branded medicines have the same active ingredients, however inactive additives such as binders, fillers, etc. may be different. It is possible for bioavailability to be compromised thus impacting on efficacy. The FDA takes all these into consideration before labeling generics as equivalent to branded medicines. For instance some nifedipine and diltiazem of equal strength and dosage form are not considered equivalent by FDA because of differences in bioavailability. There is always the problem of quality to be considered too.

Evidence of bioequivalence is the main way to ensure the quality of generic medications. If manufacturer of generic drug showed bioequivalence, you will be confident in quality of that drug.

I am in full agreement with Ali 

I  agree with Ali. Generic drug products cannot be different from the innovator. The required quality standards are exactly the same (at least within Europe, FDA, Canada, WHO).