It's not that unusual for the two sets of connections on the back of an outlet to be used to connect the wires coming from the breaker panel to the wires going to other outlets on the same circuit. However, as you've noted, it's also possible that the wires were on completely separate circuits. If they were on separate circuits, then there are negative consequences to leaving them connected. The negative consequences can be anything from breakers that are immediately tripped (which indicates a hazardous condition) to various other hazardous conditions which could be harmful/annoying now and/or hazardous in the future (various possibilities are discussed in other answers).
Ultimately, the only way that you are going to know if the circuits were connected is to test. You can do that testing by either testing the circuits as they exist in the home (which can be quite inconvenient), or, assuming the outlets you were replacing were functional, you can test the replaced outlets to determine how they were actually connected.
Testing the old outlets
If you still have the old outlets which you removed and you have a continuity meter (e.g. an ohmmeter, or Ohm meter; multimeter; etc.), then you can test and be certain whether the original outlets had the top and bottom plugs connected together or disconnected. Testing the old outlets will allow you to be 100% certain if the top and bottom sockets were connected prior to you doing work.
If you don't have a continuity meter of some sort, then you can probably get assistance from someone at a local hardware store to test the outlets. Just be upfront with the person helping you as to what you are wanting to find out. Most such stores will provide assistance.
However, IMO, you should own something that will serve this testing function if you are wanting to do this type of work. Testers which will perform this function can be quite inexpensive. Having something to do this type of test can resolve many types of situations.
If you have a meter of some sort:
With no wires connected to the back of the outlet, check the continuity (i.e. resistance) between the connections in the same position in both the top and bottom plugs. You can do this either by using the screw connections where the wires were actually connected prior to removal, or you can insert the probes for the tester/meter into the holes where plugs would normally go (i.e. actually insert the test probes into the holes in the face of the outlet, where you'd insert normal plugs, but between where you'd insert two different plugs). If inserting into the socket slots, using the test probes available on most meters, you will often need to wiggle the probes around to actually make contact inside the slots the plugs go in.
If the upper and lower sockets are connected, you should easily read 0 Ohms across the connections for the top and bottom sockets. If they were disconnected, then you should be unable to read a low resistance between at least one pair of slots/screws on either the left or right side of the outlet.
Ideally, you would do this testing as you removed each individual outlet and either break off the tabs, or not, as indicated for each outlet as you were replacing the old one. Given that you have already removed the outlets and almost certainly have not tracked which outlet came out of which electrical box, doing this testing now will only tell you A) If all of the outlets have the top and bottom connected, then there's no need to worry; or B) the number of outlets which you need to identify and break the tabs off of; and/or C) that you need to break the tabs off of all of the outlets.
If your situation is not (A) or (C), then you will need to go through and test each location where you replaced an outlet to determine which one had the top and bottom on separate circuits. It will be quite helpful to have an appropriate tester in order to do that. But, at least you know from your testing of the removed outlets how many of the outlets you need to positively identify as the ones which need the tabs broken off (i.e. if only one of the old ones had the top and bottom disconnected, you only need to find one box where that needs to be the case and can stop investigating once you've found it).
