Snakes are normally in hibernation (called brumation) during winter months and likely do not urinate, though we are not entirely sure. Snake urine is similar to birds' — not much liquid, mostly white uric acid. You could look for that. We wonder why you ask. Snakes usually come in homes because there is food, such as mice. Mice do not hibernate and actually urinate often, within specific parts of their nest (which you can smell), but also while they are running around. If it is warm enough in your attic, both may be active. Set several mouse traps baited with peanut butter. Once warm weather arrives, plug gaps where these guys are coming in.