The easiest way is to choose which of the (up to) eight trains a day you wish to travel on and phone VIA. There is, however, a way to determine which trains are supposed to be LRC, HEP, Ren or "mixed." When on the booking page which shows the eight trains, select "Business" (or any fare category) for any train. It will open up and show some small icons. The one on the far right is "i" so click on that. On the information that appears at the top of the box it will say, not too far from the end of the line, what equipment is expected.
I just checked all Toronto - Ottawa trains in both directions for a week today. It shows all as LRC. From my experience this information isn't always correct. There is a Toronto-Montreal train which has shown "mixed" for some time. That usually means an LRC Business car and HEP coaches, but that train has been all HEP since the beginning of June. As you are aware, there can be some disappointing last-minute changes. People who have booked a single seat can be quite annoyed when they get one of the old clunkers with its "2+2" seating.
One other thing to watch for is the new seating, which is called "50-50" because half the seats face one way and the other half face the other. The information that VIA agents have cannot be relied on. Sometimes they know which trains have this configuration (and a few trains are running regularly with a locomotive on each end so they are not turned at their destination) but many times the computer doesn't show this. About a week ago I boarded a train in Toronto and asked the agent in the Business Lounge to show me the seating plan on the computer. It showed the regular "2+1"seating. When I boarded the train a few minutes later I saw that it had the "50-50" seating in the economy coaches as well as the Business car. Many people cannot face rear-ward and others are not pleased to be sitting face-to-face with a stranger.
If you wish to be sure to be facing forward - and not part of a four-seater - you should choose double-seat rows 11, 12, 13 or 14. Row 15 permanently faces the rear as it is meant for passengers accompanying a passenger in a wheelchair which is tethered adjacent to that. Rows 2 and 3 are a four-seater no mater what the configuration. Rows 8 and 9 will be a four-seater if in the new configuration. On the single seat side there are three pairs of facing seats. If the train is one of the few that is not turned then there is no way of knowing which way any seats are facing. The computer seating charts available to VIA staff is usually accurate in this regard.
VIA has promised that some day soon seating charts will be available on their on-line booking site. About six years ago I was told that this facility was coming in six months!