The name of the murder can give a big hint as to who the killer is.

For example, I recently had a case called the "Asset Stripper". The murderer was a coworker of the victim.

This can be the first step in finding strong leads to solve a murder.

Calling cards are a straightforward lead. Usually being a crumbled note or business card or literal writing on the wall.

Again, using a recent case as an example, a business card, saying something like, "THEY DIDN'T HAVE WHAT IT TAKES..." was found on the scene near the corpse. Business cards are often linked to coworkers.

Oh, and make sure that you scan notes or cards for fingerprints. They very often, if not always, have the killer's prints.

This can be a good start, but I often use this as the last lead if no other leads have helped me.

Every city should have somewhere to buy weapons, whether you're buying a simple box cutter at the local hardware store or getting a rifle at the illegal weapons shop. These places will have a sales record somewhere. There's usually the physical one (I prefer this one) and the digital one. Just note down the names, initials, etc., and now you have some leads.

This one hasn't always worked for me, but it is a good lead. There have also been some cases broken open by this lead.

I don't have much experience with kidnappings, but I have found that the note to the victim about where to meet the kidnapper is often my best clue.

It has a time and location. Travel to the place and check the camera feed, specifically at/around the time of the meeting. Is there someone with your victim? That's most likely your kidnapper.

I prefer to run up the stairs to the crime scene no matter which floor it is. While running up the stairs, I will question every person I half-bump into about if they saw anything. Usually, someone will have seen something. This often is like the stalking and harassment notes, but that doesn't make it any less useful.

People will often give details like hair, height, or build. Another thing that people can tell you is if the suspicious person is/was drunk.

There is an exception to someone seeing something. In a murder case where the victim was shot with a rifle, there is a chance that the killer was never in the person's home and shot the person through a window (or wall, in some cases I've seen). This causes me great mental pain when I finish running up 9 flights of stairs, bugging every living thing on those stairs, just to find the person was shot through their window by a rifleman.

Despite the exception, this is still one of the best leads in my opinion, as it gives the player some things to look for in their suspects very early in the case.

This won't work with all cases, but it's helped me with many cases, mostly infidelity cases.

In my experience, the #1 type of case this helps with is infidelity. The affair partner will be listed on the cheater's half of the book. Depending on the information given by the spouse, you might only need this book and a cruncher in city hall (I recommend medical wards' reception cruncher). Just begin punching in every name on the cheater's side into the government database and line up features with your info.

It can also help if you find a poem or letter with only the affair's initials. Line up any person with the same first initial and start searching their apartments (usually just 1 to 3 in my experience). You can also use this in tandem with the government cruncher.

This usually doesn't help me with murder cases, but there are some types of murder cases it helps with.

If there is a computer belonging to the victim/cheater/etc. on scene, make sure to log in and check the V Mail. These can have worthwhile clues, such as, for murder cases, messages relating to stalking or harassment with short descriptions of the killer (often about build, hair, or height).
In infidelity cases, there can even be direct evidence of who the person is cheating on their spouse with.

So, always check the V Mail, even if it doesn't have anything (sometimes it won't), it's better to check than to miss vital clues.