A few ideas:
You might try to follow the example of the composer Hindemith, who was reputed to be able to play every standard orchestral instrument. He was certainly an exception musician, and he clearly devoted his whole life to music. Whether you want to go to that level of dedication is another matter!
It would be worth investing in a few books about learning to play various instruments. For example, a few days reading books about learning to play the violin, together with knowledge of the tuning of the 4 strings of the instrument would give you a reasonable grasp of what "ought" to be playable. Clearly this would not give you enough detailed knowledge to write the solo part for a violin concerto, but knowing that the violin can play pretty much any melody that does not go below G below middle C ought to be a decent staring point.
Having got a reasonable understanding for the violin, it would not be too difficult to get a similar level of understanding for viola, cello and double bass.
For woodwind instruments, the most important restriction is to look at the lowest and (normal) highest note they can play, If you stick to a melody line that lies between those 2 limits and only plays one note at a time, you are probably fine.
Brass might be similar, but I am no Hindemith and have very little knowledge of brass instruments. Maybe a book or an Internet tutorial on learning the basics of French Horn, trumpet, trombone etc would be enough.
To give an example, I am not a flautist, but knowing that the flute can play any note between middle C and at least 2 and a half octaves above (and probably more), and has virtually no limitations about playing a large jump from one note to the next, then almost any melody line within a range of 2.5 octaves above middle C ought to be possible for any decent flautist.
It might well be worth the effort of going through the music theory syllabus for an exam board such as ABRSM, as they require knowledge of many orchestral instruments, and go into some slightly obscure areas such as transposing instruments like the B-flat clarinet. The syllabus will not tell you everything you need to know, but it would prevent you making some basic errors at least.
Finally, you could have a look at the very large number of orchestral scores available for free on IMSLP and see for yourself the sort of parts that many composers have written for instrumentalists.