I found I was quite harsh to begin with almost expecting CPL standards because that was what I was used to producing myself. Also you won't have a clue what the average range of ppl standard of flying is susprisingly high in some cases and shockingly low in others.
Then you sort of develope over time a gut feel.
Its a bit like when you send a student first solo.
You never really think about it after a while. Some peoples checks are once round the circuit others get a bit more attention.
The worst ones in my experence are the people that just stay in currency and no more. So if the check is every 28 days they will turn up on the 27day and do 0.5 of circuits. Then they go on holiday and need a check ride. If you give them anything other than circuits the fun starts.
And there are some bizarra practises out there. You have to separate the way you do it and teach it and what seems strange but the end result works.
And the trick is as well is to get them to fail themselves if they are really bad.
Most people know in themselves quite quickly that they could do with a bit more. Which is why its important to really keep your hands to yourself unless you really really have to save the day. It has to be obvious to the pilot that you had to take control. And in the past with me its been "your fine" to a border line case and next time you look at the booking sheet they have a hour booked dual. And if you approach the whole check with the right attituded its rarely a problem. And if you get a good reputation for being helpfull and supportive you will get more bookings because of it.
If it does become a problem step back and pass them to the CFI or an unrestricted instructor to sort out.
I might that this subject was completely lacking in my FIC so I know exactly where you are coming from asking the quetion.