Lots of car seat manufacturers have Fit Finders, some also call them compatibility lists. These lists check if the car seat is compatible with your car and which vehicle seats to place the child seat on to.
All you have to do is know the make, model and year of the cars you are planning on using and you can then check if it appears on the compatibility list for the child seat.
Compatibility lists check features in your car such as a suitable seatbelt is present, the ISOFix is present, the ability to accommodate that size of child seat, top tethers etc.
But please be aware it doesn’t do everything. So for example, it isn’t able to confirm the length of the seatbelt, the ability to fit more than one child seat, if the driver needs additional leg room and therefore reducing the space in the rear seats or that the seat can actually rotate when locked in position. And it doesn’t crash test that seat in the chosen seating position in that vehicle.
So we would always recommend that the compatibility list is the starting point and the physical fit is always recommended to ensure physical compatibility. This is where you should use the help of a retailer, who can do this for you.