Quick fix 1
Wobble the dice in the palm of your hand. You can feel how the hidden weight wants to settle at the lowest point and work out which face(s) the die is loaded to favour.
Quick fix 2
If your dice are loaded with iron, you can use a magnet. If the magnet is attracted to the centre of one face, then the die favours the opposite face; if the magnet is attracted to an edge, the die favours the two sides adjacent to the diagonally opposite edge; if the magnet is attracted to a corner, then the die favours the 3 sides adjacent to the diagonally opposite corner.
When you have a lot of dice to distinguish from each other
My dice are often used to teach statistical analysis – the dice are deliberately mixed up and the students have to identify which is which by analysing the results of many rolls. In this case I recommend the teacher secretly marks the dice beforehand. A tiny pin prick in the centre of the spots will escape notice unless you are looking for it. Better still, use a UV (ultra-violet) pen. The ink is invisible in normal light but glows brightly when you shine a UV torch on it. Both pen and torch are cheap and easily available, from eBay, for example.
Here is a method which can distinguish between the dozens of types and hundreds of variations of trick dice I supply. Mark the pips according to the following system:
Position the die with the 6 on top and the 5 facing you.
Mark the 5-face according to the type of trick dice.
Mark the 6-face according to the favoured numbers.
Here are some examples ….
Lead face-loaded 6
iron edge-loaded 1,2
You can combine the marks on the 5-face to identify other types …
Overloaded 3. It contains both a lead weight and a hollow.
Magnetic Core. The whole die is magnetic, not just one face.