We just had a vehicle stolen and we have an iPad in it. I don't think we have the Find My App turned on. Is there any way to find it? And if not, how do I mark it as lost?

If Find My is not turned on, then there's no way to track it.

If Find My is not turned on, then you also cannot mark it as lost.

Find My is what allows you to do all these things. If for some reason you chose not to haver it activated then the iPad is not likely to be recoverable.

Link-> If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support