Personally, I like to take a handful of d6s and roll them at random intervals. Different number of dice, change up the timing, etc. Let the players see me rolling. If they ask what I'm rolling for, say "Oh, don't worry about it" and give them a Monty Burns grin.
Now, if the issue is different dice pools for different guards using different vision mods and different players using different methods of stealth, I've got an easier solution for you. First, get a DM Screen, so the players don't see what you're doing. Second, find an online dice roller, and have it roll a couple hundred d6s or so, and record the rolls (legibly) on a piece of paper. When a perception check is needed, take the next [dice pool] entries on the list, and that's your roll. Third, roll some dice behind the screen as cover, so the players don't catch on.