How do you know how much weight is too much?

You could use "Space Engineers Calculator" an app that calculates thrust needed for your BP or a list of Blocks filled with any item.

I wrote a comment in another thread about this topic a few weeks ago, and I wont bother repeating everything, but here's a few quick notes (and some stuff I forgot to mention earlier):

• For a rough starting figure to plan around, I just double the intended cargo load. That is: if I'm planning for a miner to carry 60,000 kg of ore as a standard load, then I anticipate it's loaded weight is going to be around 120,000 kg and use that number for planning thrusters. At sea level at 1.0 gravity, it's often possible to load the same ship to around 180,000 kg (though it's going to be very sluggish at that weight and you have to fly it very carefully), so I also keep that in mind when planning storage. (Above sea level, you're going to be able to overload it less than that depending on the altitude.)

• For vertical thrust, I tend to plan around 5,000 kg for small atmo thrusters and 30,000 kg per large atmo thruster.

• For cargo capacity, the numbers I plan around are 10,000 kg per med container, 45,000 kg per large container, 3,000 kg per cockpit or connector, and 6,000 kg per drill.

• Note that none of these numbers are perfectly optimized. They're just quick and dirty numbers that give me a place to start.

• For front, back, and side thrusters, take the sin of the angle you anticipate tilting at and multiply that by your vertical thrust. For example, I tend to use 20 degrees as my max cutting angle, so I'll put sin(20) = ~1/3 of my vertical thrust pointing forwards and backwards, and about half that pointing to each side.

Test them in a solo game in survival with creative tools first.

If you don’t want to test them first, build them with plenty of thrust and power to spare. Also do the first mining run next to your base, so you can easily just slap on more thrusters or batteries.

Alternatively, take out a container or two so they don’t get too heavy.

Or make a cargo rover that you unload several mining runs into.

As others have said you can calculate lift force yourself, test with expected loads, etc, or if you can use scripts (some servers don't allow them) try scripts like: ExcavOS

Alternate option to those already posted.

Take your ship into a creative world. load up up to the max and fly it around a planet with SLIGHTLY higher G than the planet you actualy play on. See how it actualy flies and performs.

Also test how well it lands.

You can derive this from the formula Force is mass times accelaration. You can look up the amount of force each atmospheric thruster has (https://spaceengineers.fandom.com/wiki/Atmospheric\_Thruster) and add together the individual forces for a total force that you are able to generate. Then you divide it by the accelaration due to gravity (9,81 m/s^2) and that will give you the amount of mass you can lift (including your ships mass).

I like to build a "drydock" which has multiple contact points to ensure its safety during thruster testing.

I think there's a way to calculate it... I know that once not long ago I did a thrust comparison Google search and found a chart that showed all the thrust amounts for each thruster type and size.

Here is the link: https://spaceengineers.fandom.com/wiki/Thruster_Comparison

Edit: Also keep in mind that your power output needs to be up to the task as well. If using thrusters in multiple directions, the power needed might exceed the output capabilities if you don't have enough batteries. It might be necessary to add batteries to allow for that. (I've never needed more than three for a flyer.)

https://343n.github.io/spaceengineers-thrust-calc/

Math, calculator, experience, or controlled test. Those are your three (edit: FOUR. I r can count) options.

Math and calculator have been covered by other comments, and experience is just fiddling with SE ships until you have a feel for it.

Controlled test: Right next to base, load up your ship with stone/ore/steelplate and:

1. See if it can lift off the ground.
2. Move your ship around; do your gyros have enough force?
3. Gently roll it forward/back/side-to-side and see how far you can tip it while loaded. Do your off-vertical thrusters have enough thrust for gravity/what's your max inclination in gravity?
4. Accelerate to 10-20m/s and see how quickly you get up to speed. Try to stop from that speed and see how quickly your dampers bring you to a stop. Do you have strong enough retrothrusters? Do you need to flip for a counterburn using your main forward thrusters?
5. Do a 3-axis burn and watch your power meter: do you have enough energy to run 3-axes of thrusters at full power at the same time? (this is more common that you might at first think)

And always. Aim. To. Miss.