Aluminum is a lot lighter. And is not magnetic.
Galvanized has fairly unique surface look with the zinc coating.
Stainless is generally shiny.
The really sort out steel alloys you need hardness test data, tensile test data.

without getting complicated
A file is what you want to carry around (or substitute file for some sort of scratching device, a 6 inch steel ruler works well)+ a magnet helps

Magnet Test
Aluminium - not magnetic
Steel - magnetic
Stainless - usually not magnetic, but some grade are...

file / scratch test
Aluminum - is soft / easy to file / scratch
Steel - is in the middle (except for tool steels)
Stainless - is hard / hard to file / scratch

Without carrying around expensive equipment, a 6" steel ruler + a small magnet can be left in your pocket all day long and is easy to use
There is always exceptions but this will get you right 95% of the time.

Andrew O'Neill
Specialist Mechanical Engineer
Australia

Also, spark testing.

https://en.m.wikipedia.org/wiki/Spark_testing

1. Visual Inspection:

Color: Aluminum typically has a lighter, silver-gray color, while steel can vary in color depending on the type and surface treatment. Stainless steel may have a more polished and reflective surface, while galvanized and galvannealed steel can have a dull gray or matte finish.

Surface Texture: Aluminum often has a smoother, shinier surface compared to steel, which can have a rougher texture.

Reflectivity: Aluminum is highly reflective, while steel is less so, especially when it has rust or corrosion.

Grain Structure: Steel may have a visible grain structure on its surface, which is not typically present on aluminum.

2. Magnetic Properties:

One of the most straightforward methods to distinguish between aluminum and steel is to use a magnet. Steel is magnetic, while aluminum is not. So, if a magnet sticks to the material, it is likely steel. If the magnet does not attract the material, it is likely aluminum.
3. Weight Comparison:

Aluminum is significantly lighter than steel. You can compare the weight of the object in question to the weight of a known piece of aluminum and a known piece of steel of similar size to make a preliminary assessment.
4. Conductivity:

Aluminum is an excellent conductor of electricity, while steel is not. If you have access to a conductivity tester or a multimeter with a conductivity setting, you can test the material's electrical conductivity. Aluminum will have high conductivity, while steel will have lower conductivity.
5. Spark Test (for Steel Varieties):

If you want to differentiate between different steel varieties (e.g., stainless, galvanized, and galvanneal), you can perform a spark test using a grinding wheel. Different steel alloys produce different spark patterns. However, this requires some experience and reference materials for comparison.
6. Chemical Tests (for Advanced Users):

If you have access to a chemical testing kit and are knowledgeable in material analysis, you can use chemical tests to identify specific metal alloys. For example, you can use chemical reagents to distinguish stainless steel from other types of steel.
7. X-Ray Fluorescence (XRF) Spectroscopy (for Advanced Users):

XRF is a non-destructive technique used in materials analysis. It can provide precise information about the elemental composition of a material, helping you identify specific metals and alloys accurately.