What do you normally use for that -- a percentile die? And how often do you roll? If either of you would like to go into specifics, I'd be much obliged.
It varies depending on the system honestly, for Cortex rules I usually use a d12 and roll under their helpfulness rating of around 7 for Jane/Joe average hired help and temporarily lowering the rating by one point for every time I successfully roll their help check that session.
For simplicity sake (I assume you're using d20/Pathfinder?) I would suggest giving the hired NPC's a helpfulness rating of around 11 and then roll under it with a d20 to determine if they have useful advice or not in a given situation. (The lower/higher they roll the more or less pertinent/insightful their advice will be, with a botch having them get their facts completely wrong. "I heard werewolves have a secret weakness to gold as well as silver, only a hundred times worse, maybe using some gold coins in a sling shot will kill it quickly or dipping your blades in melted gold?")
As for how often I roll, it depends heavily on the situation, with out of combat inquiries, (I.E: Should we post guard for the night? Do you know of a good place to eat in this city?
) usually not warranting a roll and just being made up based on the NPC's personality/past experience.
Tactically important choices, (I.E: Should we ambush the bandits from the trees with bows or try to pretend to be helpless travelers to lure them into a false sense of security?
) Usually call for a roll if one of the options would be clearly superior to the other. (Such as with the former example posing as helpless travelers will simply get them surrounded with crossbows and weapons pointed at them, whereas a tree ambush would take the bandits by surprise.)
On a good roll the NPC may lean towards the better choice based on personal preference, on a poor roll they favor the poorer choice for whatever reason I feel is appropriate. (Maybe they have a dislike of heights, don't want to get covered in pine pitch, or just not thrilled about giving the rogue an upskirt view of their naked nether regions.)
In combat the helpfulness rating is usually rolled under to see if the NPC notices one of the players is in trouble (being flanked, ganged up on, etc..) Success = the NPC coming to their aid, failure = they were too caught up in their own fight to pay attention to anything beyond keeping their insides inside them.
For in combat tactics the NPC's usually don't have much to offer beyond yells of "Flank my target!"
or "The little one near the bushes has a crossbow!"
etc.. (Trying to plan a meticulous strategy in the middle of melee combat is something I rule is generally impossible for everyone involved given most people can manage perhaps a short sentence or witty one liner every time their action turn comes up with how fast paced combat is supposed to be taking place.)
For NPC's that are either dramatically smarter (or dumber) then the average person of their class they can have a helpfulness rating a few points above or below the average middle die result (I.E 10 being "average" for a d20, 50 for percentile etc..)
If you want to get particularly in depth you can give them more then one helpfulness rating depending on their character class. For example a ranger may have a general helpfulness rating of 8 on a d20 and a separate wilderness helpfulness rating of 14. This lets the NPC shine more in their area of expertise rather then being equal in all aspects of aiding the party. (It also encourages the players to seek out a specialist NPC to make up for their weaknesses as opposed to "whoever is cheapest.")