To determine Tesco's Weaknesses use SWOT analysis, that's what the the W in SWOT stands for.
One of Tesco's weaknesses has been its failure to dominate the city centre marketplace. Although it has some prominent city centres stores, e.g. the Carlisle store in the North of England, it has failed to make them universal. For instance, Milton Keynes has no city-centre Tesco store.
In the case of Milton Keynes, Sainsbury has a monopoly amongst the large supermarkets in the city centre. Tesco's failure to provide competition shows a significant weakness in its marketing approach. Given Tesco's overall market dominance, the other supermarkets can draw some relief from such signs of weakness.