Sppons,
It is difficult, because the actual intake of a secondary modern vs a comprehensive can be quite similar, or even reversed from what is 'expected', especially if the secondary modern is in an affluent leafy area, with only a few children going to grammar and the comprehensive is in an area which is poorer socio-economically and where educational aspiration in the community is low.
The easiest way to think about this is to think of 'academic ability' amongst the population of a specific age having a 'bell curve' normal distribution. Some of these bell curves will be skewed to the higher or to the lower end due to socio-economic factors.
In a true comprehensive area, the range of abilities of the intake will approximate pretty closely to this bell curve. Some children, predominantly at the lowest end, will attend Special Schools. Some affluent children, often skewed slightly to the upper end, will attend private schools. However, the children within the school will be of a full range of abilities.
In a grammar area, the populations of the grammar schools and the 'other' school [never now named 'X secondary modern school', because of the negative connotations, but in practice that is what they are] will differ markedly from the bell curve. In the 'other' school, removing those who pass the 11+ will produce a sharp drop in the numbers of higher ability children vs the bell curve. As the 11+ is notoriously imperfect at measuring actual ability, there will be a 'tail' of children towards the upper levels of ability, but there will be fewer than there are in the general population in that geographical area. Equally in the grammar school, the 'peak' of the distribution - ie the ability with the largest number of children - will occur at a higher ability than in the general population (how high depends on its selectivity - from top 25% in Kent to 2% or less in superselective areas). there will be a 'tail' of children with lower abilities, but it will fall off much more rapidly than in the general population.
A school in the same town as a grammar cannot, by definition, be comprehesive BUT it does depend hugely on how selective the grammar school is and the proportion of pupils who go there. m,any posters on previous threads have compared e.g. Cheltenham - 1 very highly selective grammar, several near-comprehensives - with Gloucester - 4 grammars, 1 faith school, a number of very obvious secondary moderns.