How do one know if one is running out of memory space for attachment for the forum?
You can check it in your user profile by clicking your username. It is stated under "Upload Quota".
What should one do? Delete old attachment? If yes, how to find one post with attachment instead of going thru each post at a time?
We recommend you to delete the old and outdated one, such as those image that you post in the Buy and Sell section. You can use the "User Attachment Control Panel" to view and delete your unwanted attachment. It can be access by clicking the link "User Attachment Control Panel" in the user profile.
Can Singastro increase memory space? or use album link to reduce memory space
You can send your request to the admin to increase your quota if you really need it. Anyway, we strongly recommend user to upload astro/user images under the album which had no specific quota.
Have a nice day.