I've been trolling Lake O for 15 years without downspeed. Too many life events recently to make the $1,000 investment in a probe. I've had many days with insane numbers. Just pay attention to the angle of your rigger cable and listen for the hum. Or, as mentioned above, look at the bend in the dipsy rods as well.
Downspeed is super important but not having it won't break your day if you pay attention to some other details.