I notice that some kosher places have a mashgiach (kashrut inspector) that vsists the place occasionally. Others have someone there permanently.
What criteria is used to decide if a place needs a mashgiach temidi?
Given that many operations require complete knowledge and supervision of the processing, then a mashgiach would need to be present whenever ingredients are brought in to the plant and would have to be able to inspect the invoices as well as the ingredients. Since a restaurant has constant production, then it would need a mashgiach tmidi (though there may be circumstances in which the owner might be trusted as a mashgiach tmidi) in order to avoid problems.
There is a story in which a hotel color coded its utensils blue (dairy) or red (meat) with spray paint on the handle. A mashgiach managed to stop one of the workers from (innocently) respraying some silverware when they needed some more. It was not a deliberate attempt to cheat, the worker just did not realize that it was not the color that was significant.
For example, the Star-K explains what is involved with fish fish The Tell Tail Sign
The great posek Hador, R’ Moshe Feinstein, tz”l, writes in his responsa that the proper way to given kosher certification to fish productions is for the mashgiach to see every fish. Therefore, Star-K certified salmon, as well as all other fish varieties, have a mashigiach t’midi who examines every fish during production.
The mashgiach has to position himself strategically in order to be able to see all the fish before the skin is removed. Having the workers check each fish individually is not sufficient. The mashgiach must set up a system which ensures that each fish is inspected by him.
To meet the requirements of bishul Yisroel, the mashgiach puts on the steam after the fish has been rolled into a cold oven.
After cooking, the mashgiach must be present for the canning and labeling. In addition to the can code, a specific kosher code is added as a siman (sign) that the fish was produced and canned in the presence of a Star-K mashgiach. The producer’s labels, complete with the certification symbol, have to be in the possession of the mashgiach so that they would not be inadvertently used for a non-supervised run.
Every kashrut authority has their own criteria as to which places need a mashgiach temidi. For example, they may decide that if the owner him/herself is Shomer Shabbos they can be trusted and a yotzei v'nichnas (mashgiach who comes every so often) is enough. Or that the type of establishment determines e.g. a restaurant might need one, but a bakery might not (the mashgiach comes every few hours to sift the flour and take challa etc.). This might be a reason to prefer or not rely on a particular kashrus organization - if they are more willing to give kashrus certification without a mashgiach temidi.