Just offer it and see what happens.

What works for me is this. I start with the prey in a zip bag and place it in the sink with something on it to submerge it completely in the water. I think let it thaw. This time will vary by prey size but 30 to 45 minutes should be more than enough for a large mouse. Remove the prey and squeeze its head and guts, they should not have any cold or cool spots. If they do submerge a bit longer.

I then drain the water and again fill the sink with the hottest water I have in the tap. I again submerge it for 15-20 minutes to get it warmed all the way through to represent a real live rodent.

You may not need to thaw as long. This is with all my feeders in the sink including a XXL rat. Just check after 15 minutes and see how it is.

Live rodents have a body around 100F so you feeders need to near that for best results. Once pulled from the water feed quickly, they start to cool pretty fast.