It isn't easy.
Since a lycanthrope's true form is humanoid, spells and items that reveal their true form or revert them to that form won't work, which removes most of your options.
There are a few left, though, primarily spells that function based on creature kind (specifically "shapechanger"). Some specific examples would include locate creature, glyph of warding, and symbol. (Locate creature may not work, as it doesn't detect creatures that are "in a different form" from the named creature. I assume if you say "were-rat", it would only detect the hybrid form, considering human and rat to be "different forms", but that's likely to be a DM call.) It depends somewhat on the situation, but you will probably need to do some work to isolate your suspect to be sure he triggered the effect and not some other shapechanger who happened to be nearby.
You could hit the suspect with one of several effects that shapechangers are immune to, such as the polymorph family of spells, but there it's hard to tell if the target is actually a shapechanger or just resisted the spell with a successful save. And going the other way, a lycanthrope could, if they realized what was happening fast enough, willingly allow themself to be polymorphed, thus throwing you off.
On the magic item side, the obvious one is an item with the Sentinel minor property (DMG p.143), tuned to Shapechangers. If your sword is glowing blue, then there's shapechangers about. Finding such a specific item would probably be a quest in and of itself, though, and it may be difficult to pick out a specific person, since the Sentinel range is large enough to encompass several small buildings. Isolating the target is, again, probably a necessary step.
The alternative would be various interrogation spells like zone of truth or detect thoughts, though you'd likely have to be fairly overt about it. Just directly asking "Are you now, or have you ever been, a were-rat?" is likely to alert him that he's been discovered!
Your best bet is probably a glyph of warding set to target shapechangers. If you set a Spell Glyph with a no-save targeted spell that has a visible effect (magic missile sounds good), then you can watch and see if it goes off when he passes by. Ideally, set it in a place where only your suspect will go so you can be pretty sure it won't inadvertently go off early just because some other shapechanger stumbled into range. You can do much the same thing with a symbol, watching to see whether it goes off and (if there's a group) who exactly got zorched by it, but that is of course a much higher-level spell.
The old standby: Violence!
I assume a lycanthrope in human form retains their damage immunity to non-magical weapons that aren't silvered. If they do, then a possible answer, harsh as it may be, is to cut or stab your guildmaster with a mundane weapon. If it fails to cause a wound, or the wound immediately heals over, then you've (probably) found a lycanthrope. The down-side is of course that if you're wrong, you probably get arrested for assault even if you magically heal the injury afterward and say you're really, really sorry.
