Not all areas have catchments, but for the places which do you are likely to be in the priority admissions area for only one school.
Those maps must be readily available with all other admissions information .
But, being in catchment might not be enough, if you are towards the edges of it, and it's a bumper year for in-catchment siblings.
So you also need to look up the effective admissions distance for each nearby school. This infimshoukd be on the school or LEA website. But do bear in mind that this is much more changeable year-to-year (changes to schoo reputation, new housing built nearby, lots of siblings a couple of years after a bulge class etc). So treat these figures with some caution, and try to find out info for the past few years to show trends..
Also, remember that they are based on offers made on National Offers Day, not on who actually starts in September.
I can't recommend an app/website that has good quality information, other than your local LEA/ schools themselves. All infomalso needs to be read in conjunction with what the entrance criteria say.