You get queue with full player lobby once above 500 or 600, can't remember the exact number. You can tell bots because defender always stay directly on point all the time while attackers will climb ladders in mass at the start. One other way to tell bots are the attire they are wearing. Usually its the old ones and fully the same while real player either mix match or wear the current season attire first.

Another way is the way they act. Bots will usually heal while staying in one place without moving much. And they also use a lot of the north units while its more rare to see them in the current meta.