Well, if they are Martians, and if they do have hearts, they'd almost certainly beat at a slower rate. Their veins might be distended.
10 seconds sound clip from the The War of the Worlds (1953) movie soundboard.
You can hear this line at 00:22:29 in the DVD version of the movie.
Quote context
[...]
- Oh, it's possible. At least it seems certain they're from some planet other than our own.
- Suppose they are Martians, professor. What would they look like? Bigger than us? Smaller?
- Then you think they'd be breathing creatures, like us.
- What about the heart and blood and all that?
- Their senses could be quite different from ours.
- They may, for instance, be able to smell colors.
- Precedent in our own evolution makes it possible that they have more than one brain.
[...]